A stock trade isn't made and a commission isn't earned because the markets are closed. A bond isn't sold because a rating agency's computers don't work. A shipment doesn't move because flights are operating on schedule.

Small problems such as these, the fallout from the attacks on the World Trade Center, could cascade into big problems for the U.S. economy, experts say.

"Some business is lost forever," said Mark Vitner, an economist at First Union Securities in Charlotte, N.C. "At this point, a lot of business and household decisions are being postponed. The longer it goes on, the deeper the problems for the economy. "

Vitner said there's no question that reduced activity as a result of the attacks will cut economic growth in the short term, possibly driving it into the negative range for the third quarter.

Many businesses that operated in the Wall Street area are still reeling from the damage caused to buildings, computers and utilities by Tuesday's collapse of the trade center towers.

The computer operations of both Moody's Investors Service and Standard and Poor's were still down Wednesday, as were the cash transferring operations of the Bank of New York, one of the nation's largest processors of securities and payments.

New York and Chicago stock markets weren't expected to open before Friday at the earliest, although the Chicago Board of Trade and Chicago Mercantile Exchange plan to resume electronic and floor trading on Thursday.

Still, the biggest challenge may be overcoming the drop in confidence caused by fear.

"You have to keep your eye on the consumers," said Gary Thayer, chief economist at A.G. Edwards & Sons Inc. in St. Louis, Mo. "If they don't feel comfortable going out to the malls or going to public events like baseball games, these are things that will subtract from the economy."

He added: "The same applies to business. If people feel they can't travel or feel that security measures are so severe they don't want to travel, it could be a drag."

Most airlines were grounded after the attack, and the Bush administration said Wednesday it would continue the ban on air travel until safety of passengers could be ensured.

Both Thayer and Vitner said decisions to suspend major league baseball games and to close shopping malls also contributed to a potential downturn in consumer confidence and spending, a key driver of the economy.

Among the industries already feeling the pinch are air travel, hotels and restaurants, entertainment and retail.

Robert Brooks, a professor of financial management at the University of Alabama in Tuscaloosa, worries about long-term effects of the market closure.

"The people who are making their living trading stocks are clearly not making any money right now," Brooks said. "But every day that it remains closed, there's more uncertainty in the marketplace and that can only lower the value of stocks and depress things further."

No one has yet tallied the impact of the physical loss, either.

James Meiskin, president of Plymouth Partners, one of the largest relocation firms in New York, estimated that up to 12 million square feet of office space in the Wall Street area has been destroyed.

He speculates that just half of that will be replaced - a huge long-term loss in rents for owners, fees for service providers and taxes for the city.

"A lot of these firms were downsizing already," Meiskin said. "Some are gone forever, obliterated. And some are too traumatized to consider staying in New York. "