The circuit-breaker test, a response to the May 6 plunge that wiped out $862 million of share value in 20 minutes, will begin as a pilot covering five stocks before expanding next week. SEC Chairman Mary Schapiro said this week the agency is considering regulating the speed of stock orders as part of a broader effort to rein in electronic markets.

Regulators are under pressure from investors and lawmakers to show they have a grip on markets increasingly dominated by computers trading thousands of shares in less than a second. Different rules on as many as 50 American equities venues helped exacerbate the May 6 selloff as some platforms were overwhelmed with electronic instructions to sell shares.

The SEC needs "to explore whether bids and orders should be regulated on speed so there is less incentive to engage in this microsecond arms race that might undermine long-term investors and the market's capital-formation function," Schapiro said at a conference in Montreal. "The markets have to serve that function for companies to raise money, create jobs and allow the economy to grow."

The circuit-breaker test, scheduled to last through Dec. 10, will pause trading for five minutes when a company rises or falls 10 percent in five minutes or less. The curbs apply only between 9:45 a.m. and 3:45 p.m. New York time.

Nasdaq OMX Group Inc. will activate the program with all of its S&P 500 stocks on June 14, spokesman Robert Madden said in an e- mailed statement this week.