Harry Macklowe, the New York developer, was flying high in February when he decided to buy a portfolio of prime Midtown Manhattan office towers for nearly $7 billion, using only $50 million of his own money.

Mr. Macklowe was already well represented in the Midtown market, where rents were rising at a staggering rate. His 2003 purchase of the General Motors Building on 59th Street and Fifth Avenue for $1.4 billion, though derided at the time as reckless, had been vindicated as the value of the building soared, enhancing Mr. Macklowe’s reputation as a visionary tycoon.

But as the crisis over subprime residential mortgages spills over into other real estate sectors, causing a severe tightening of credit, there is widespread talk in the industry that Mr. Macklowe is in deep trouble — so much so that he could lose control not only of the newly acquired portfolio but also of the G.M. Building and other properties that were used as collateral for short-term debt that must be repaid six months from now.

Through a spokesman, Mr. Macklowe, who lost the Hotel Macklowe in a real estate squeeze in the mid-’90s, declined a request for comment. But his son, William S. Macklowe, expressed confidence that their company, Macklowe Properties, would be able to pay off the debt.

Some real estate specialists say that the February acquisition of the seven Manhattan buildings — a deal consummated in just 10 business days —will be remembered not just as a feat of financial derring-do but also as a watershed that ended two years of frenzy in the commercial real estate market.

“If you’re looking for a poster child for what’s been going on, it could well be that deal,” said Mike Kirby, a principal of Green Street Advisors, a research company in Newport Beach, Calif., that specializes in real estate investment funds. “It had all the elements of the froth in the market — assets flipping left and right at ever-higher prices and excessive amounts of debt at ultracheap prices.”

Recently, many sales have been postponed, canceled or restructured, according to investment brokers and lawyers. As with residential mortgages, commercial loans are pooled and packaged into bonds that are sliced into portions carrying different degrees of risk. In recent weeks, investors have largely shunned these securities backed by commercial mortgages.

Highly leveraged loans “are not being made or are very rare now,” said Robert O. Bach, a senior vice president at Grubb & Ellis, a national brokerage company. “So, some people would say, there’s been more of a return to sanity.”

As the credit markets tighten, capital is expected to become more expensive, causing building values to drop. So far, however, there is no evidence that sales prices have declined, and brokers say there is still plenty of money available for real estate investments from foreigners attracted by the cheap dollar as well as from cash-rich pension funds and insurance companies.

In a deal completed this month, Prudential Real Estate Investors paid $280 million in cash for the Democracy Center in Bethesda, Md., according to the seller’s broker, William M. Collins, a managing director at Cassidy & Pinkard Colliers, a brokerage firm in the Washington area.

But in other signs of how the credit squeeze is affecting sales transactions, Tishman Speyer and Lehman Brothers recently postponed the completion of their $22 billion acquisition of Archstone-Smith, a real estate investment trust that owns interests in nearly 88,000 apartments, from late this month until early October. (Shareholders approved the sale yesterday.)

And a REIT that specializes in office buildings in Silicon Valley, Mission West Properties of Cupertino, Calif., said last week that its planned $1.8 billion acquisition by a private equity company had fallen through because the buyer’s lender had withdrawn from the transaction and no substitute lender could be found.

To be sure, the leasing market in many cities has been strong, nowhere more so than in Midtown, where landlords are now asking an average annual rent of more than $81 a square foot, a record, according to the brokerage firm CB Richard Ellis. Few large blocks of space are available. The default rate for commercial buildings has remained low.

But for several months, bond ratings analysts and others have warned that competition among commercial lenders has become so feverish that many are willing to finance 90 percent or more of the cost of the transaction based on overly optimistic projections that rents will continue to rise at a furious pace. In recent transactions, including Mr. Macklowe’s, the expected initial income from the buildings was less than 4 percent a year, with cash flow projected to rise significantly as leases expired and rents reached market levels.

But in the recent hot market, said Adrian Zuckerman, a real estate lawyer at Epstein Becker & Green, “people were not buying the income stream; they were buying the building for what they could sell it for in a year or two years.”

Scott A. Singer, the executive vice president of the Singer & Bassuk Organization, a real estate finance and brokerage company, said long-established investors “have often found themselves on the sidelines while other players without a strong track record have bid up deals, using a lot of leverage.”

Among the relative unknowns who gained prominence in this market was Scott J. Lawlor, the chief executive of Broadway Partners of New York, which made its first real estate investment in 2000 by buying a former school building in Hartsdale, N.Y., for $4.8 million. Since then, Broadway has bought more than $15 billion worth of office towers, including the John Hancock Building in Boston, keeping most of them a few years or less and then reselling them for a handsome profit.

The Blackstone Group, the private equity company that recently went public, played on an even bigger scale. It bought Equity Office Properties, the nation’s largest office landlord, for $39 billion in February, and simultaneously began to dismantle it.

Without even taking possession of the buildings, Blackstone sold most of Equity Office’s portfolio in Manhattan to Mr. Macklowe in the transaction that is now raising questions. (The portfolio originally included the office portion of an eighth building, but that was later dropped from the deal.)

The problem for Mr. Macklowe is that much of the debt — $3.4 billion, according to Commercial Mortgage Alert, a weekly trade publication — is in the form of a short-term investment known as a bridge loan or preferred equity that must be repaid in February. Of that amount, about $900 million came from the hedge fund Fortress Investment Group, with the rest supplied by Deutsche Bank, Mr. Macklowe’s longtime lender. Mr. Macklowe pledged the G.M. Building and other assets as collateral.

But with credit now far less available, Mr. Macklowe is expected to have a harder time refinancing to repay the short-term debt. And typically, said Mr. Zuckerman, who was not involved in the Macklowe transaction, when the rent does not cover the debt service, the difference is added to the loan principal.

In an interview in his sleek corner office at the G.M. Building overlooking Central Park, the younger Mr. Macklowe, who is known as Billy, shrugged off the concerns and said the company had always planned to take on new partners. He said it was in “advanced discussions” with three groups that could replace the bridge loan with permanent capital.

“That’s how we always looked at the deal,” he said. The company’s next steps may also include “one or two divestitures” of some of its buildings, he said.

He said the tightening of the credit market is likely to discourage “the speculative construction that’s planned — which means that the market would only get tighter.” And in such a market, he said, high-quality buildings like the ones owned by the Macklowes would have the most appeal to investors. “In brief moments of uncertainty,” he said, “one tends to find a flight to quality in terms of investing.”

Indeed, few real estate specialists are counting the Macklowes out just yet. A lot can happen in six months, several people said.

“He always manages to land on his feet,” said Howard L. Michaels, the chief executive of the Carlton Group, a company that has helped arrange financing for some of Mr. Macklowe’s deals. “Everybody’s watching to see what he does.”