The Securities Industry Association (SIA), the trade group organizing the test, said results from Saturday's dry run were "better than expected," but the real test is yet to come.

The year-2000 — or Y2K — problem threatens to crash computers on New Year's Day because a programming defect may cause computers to read the year 2000 as 1900.

World financial markets have become so closely interconnected that a blackout in one large area or firm could spell disaster around the globe when markets open for trading in 2000.

More than 400 brokerages are checking their computer systems, aiming to ensure that the hive of computers that link up Wall Street will perform normally on Monday, Jan. 3, 2000.

On each testing day, the computers are fooled into thinking it is a date near the millennium. For instance, on Saturday, the computers were set to act as though it were Dec. 29, 1999.

In the next test, on March 13, they will be set to Dec. 30. The test schedule, which runs for five more weekends, will simulate the year 2000 on April 10.

More than 170,000 mock trades went flying through Wall Street computer systems on Saturday, the SIA said, with a light volume of calls and routine problems coming into command center, staffed by SIA workers and "coaches" from the auditing firm PricewaterhouseCoopers.

"The first day's success is important because it demonstrates that the participating firms have done their homework," said Donald Kittell, SIA executive vice president.

The test went beyond stocks, also checking systems for trading options, mutual funds, corporate bonds, municipal bonds, mortgage-backed securities, government securities and money market funds.

The SIA first ran the test last July among some of its top firms, U.S. stock markets, and clearing and settlement companies. That "dress rehearsal" went well, but now the test must scan systems that are likely to have far more faults, since small firms have less money to spend heading off potential Y2K problems.

The securities industry as a whole is estimated to be spending up to $6 billion to ensure computer systems do not crash on New Year's Day.

Kittell has said the test will be a good model as the industry readies for "decimalization" next year — when stock prices will be quoted in dollars and cents instead of in sixteenths of a point.

He said it will also help prepare for "T+1" — trade plus one day for settlement. Currently, stock trades must be settled in three days ("T+3"), but the industry is moving to close the gap between trade and settlement.