Power was restored to the Wall Street area after 6 a.m., but stock market officials had said they could operate even if the outage continued. Mayor Michael Bloomberg rang the 9:30 a.m. opening bell at the New York Stock Exchange.

"It looks like it’s business as usual," said NYSE chairman Richard Grasso.

The Dow Jones industrial average closed up 11.13, or 0.1 percent, at 9,321.69, while the Nasdaq composite index ended up 1.66, or 0.1 percent, at 1,702.00. The Standard & Poor’s 500 index rose 0.16, or 0.02 percent, to close at 990.67.

The Nasdaq stock market reported it was fully operational. But the smaller American Stock Exchange was unable to open because of problems at a Consolidated Edison substation that supplies steam used to cool the trading floor computers.

With help from a city agency, the Amex set up a portable generator and opened briefly just before its regular 4 p.m. close.

It was an important trading day on the Amex, one of the main markets for stock options. August options expired today, usually prompting a flurry of trading. However, many stocks listed on the Amex also are traded on other markets, so the exchange’s closure wouldn’t affect prices of those companies’ stocks.

Other markets opened on time, but the Bond Market Association recommended an early close at 2 p.m. EDT to give traders more time to get home. In coordination, the Chicago Mercantile Exchange planned to halt open-outcry trading on interest rate futures at the same time.

Scott Marcouiller, market analyst at A.G. Edwards in St. Louis, said the Standard & Poor’s 500 futures, which fell last night as the blackout hit, rallied as soon as it was clear there was no terrorism involved.

Trader Kenneth Polcari, managing director of Polcari Weicker, said no stocks were trading up or down as a "direct result of the blackout."

Utility stocks were slightly lower on the day, and the Dow Jones utilities average was off 0.49 at 237.47.

Treasury bonds were trading down, reversing a wave of buying late Thursday while the cause of the power outage was unknown. At 2 p.m., the 10-year note had fallen 19/32 point to yield 4.53 percent.

Many of the major movers on the stock market were smaller companies that make equipment for power transmission or backup power. Quanta Services, which repairs power lines and provides other power transmission services to utilities and telecommunications companies, closed up $1.52 at $6.90.

August is generally a slow time for the market. But with many traders unable to make it to work because of the outage of the New York subways, some of those who did reach their offices remarked at the stillness.

"I am not sure how many people are here. It’s usually much busier," said Paul Clemas, an information technology consultant for the NYSE.