As they have so often in the last few weeks, Wall Street traders came to work yesterday braced for the worst. Stock markets in Asia had taken a blow overnight. There was talk that the nation’s biggest home lender might be facing bankruptcy.

And then the Federal Reserve stepped in, with a rate cut for loans to banks and an indication of further action if the economic outlook worsens. In an instant, the despondency gave way to relief.

“The floor was elated,” said Jeffrey Frankel, the president of the Stuart & Frankel Company, a trading firm at the New York Stock Exchange. The Fed’s move, he said, showed that “our system worked.”

It was the beginning of what turned out to be a positive day. The Dow Jones industrial average rose 233.30 points, to 13,079.08, reclaiming a position above the 13,000 mark, which it had given up on Wednesday. The Standard & Poor’s 500-stock index, which had lost its profits for the year earlier this week, got them back and then some. The index climbed 2.5 percent yesterday to stand up 2 percent since the beginning of the year. The Nasdaq composite had its biggest percentage gain since last August, closing up 2.2 percent.

The problem is that no one knows whether it will hold.

“We’re down 300. Then we’re even. Then we’re up 100. Then down 100,” said Theodore P. Weisberg, a trader with Seaport Securities, which executes transactions on the floor of the New York Stock Exchange. “I don’t think anybody is comfortable.”

After Thursday’s session, when the Dow plunged 340 points only to claw its way back to level ground in the final hour, no one needed to be reminded how quickly things can change. And indeed, yesterday morning’s euphoria looked as if it might be short-lived. The market surged immediately after the opening bell, with the Dow soaring more than 300 points in the first few minutes. By 10:45 a.m., those gains had almost evaporated, with the average up only about 60.

Then the downward drift reversed itself, and the week’s trading came to an upbeat end. But as they look ahead, traders are still trying to divine whether the market has merely hit a speed bump or something more serious.

Michael J. Rutigliano, who directs floor trading for the WJB Capital Group, said the initial bounce in the Dow once the market opened was hardly a surprise to him. It seemed investors were looking for any excuse to rally, he said.

“That’s the power that raw emotion adds to the markets,” he said. “The fear has obviously been abated today, but these markets tend to be very emotionally charged. You’re going to see, I think, over the next days and weeks, swings. Because investors need to try to figure out what this all means.”

Stock prices, which are usually reactive to the news cycle, have been hypersensitive lately to anything that might indicate whether more trouble lies ahead for the economy. This week offered investors a multitude of nerve-testing events.

On Tuesday, it was a profit warning from Wal-Mart and sinking earnings from Home Depot that raised concerns about how healthy the American consumer is. On Wednesday it was speculation from Merrill Lynch that Countrywide Financial, the nation’s largest mortgage lender, could be forced into bankruptcy. On Thursday it was a broad stock sell-off overseas, remarks from the Treasury secretary and more concerns about Countrywide.

Then came the Fed’s announcement before the markets opened yesterday that it was easing its lending rate to banks and acknowledging the impact that the credit squeeze was having on the financial markets and potentially on the economy.

Countrywide itself was one of the biggest beneficiaries yesterday, as investors saw relief from its immediate peril. Its stock shot up 25 percent in early trading and wound up gaining 13 percent, to $21.43, as 105 million shares traded hands.

Within the Wall Street firms handling that trading, the recent upheaval is unnerving for even the most hardened veterans.

“I’m old school,” said Peter P. Costa, a senior floor official for Eckhart & Company, a trading firm. Mr. Costa was at the stock exchange for the 1987 crash, the dot-com bust and the Sept. 11 attacks. He said he did not think the current market troubles were as severe as any of those events, but he is taken aback nonetheless.

“You’re constantly talking to customers who’ve been saying, ‘When’s the best time to buy? When’s the best time to sell?’ ” he said. “And it’s hard.”

The market disorder has interrupted what should be the lazy days of August. At times, the workloads can be overwhelming.

“People feel like they’ve worked three weeks instead of one,” said Andrew Frankel, co-president of Stuart & Frankel. “People are exhausted. This volatility is so tiring.”

This week, the expression “thank God it’s Friday” took on added meaning. “We need a reprieve,” said Jeffrey Frankel, Andrew’s older brother. “Everybody is happy it’s Friday.”

But come Monday, all bets could be off. “Make no mistake about it,” Mr. Rutigliano said, “if on Monday some unforeseen announcement comes out, the dynamic changes.”