The Securities and Exchange Commission's plan to impose halts on some stocks that swing more than 10 percent starting Friday may be followed by more measures to slow down trading. 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. The New York Stock Exchange said it will begin implementing them Friday after delaying the program last week. NYSE Euronext said five stocks -- EOG Resources Inc., Genuine Parts Co., Harley- Davidson Inc., Ryder System Inc. and Zimmer Holdings Inc. -- will be covered by the halts Friday, according to a statement on its website. The remaining 399 Standard & Poor's 500 Index companies that NYSE lists will be subject to the rules by June 16 -- with 39 on June 14, 179 more on June 15 and the final 181 the final day, the exchange said. 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. Schapiro, speaking at an International Organization of Securities Commissions event this week, said the SEC is examining whether electronic venues known as dark pools, which don't display public quotes, are preventing stocks from trading at their appropriate prices. 'We are also looking at whether and to what extent pre-trade price discovery is impaired by the diversion of desirable, marketable order flow from public markets to dark pools,' she said. The SEC proposed rules in October that would limit the ability of dark pools to seek new orders to match existing ones without displaying prices. The agency is also considering eliminating flash orders, or pricing data shown a split-second early to a segment of market participants. Rules for stocks can't be applied in isolation, Gary Cohn, president of New York-based Goldman Sachs Group Inc., said at the conference. Since many stocks are constituents of indexes or included in exchange-traded funds, pricing won't cease entirely when trading in particular shares is halted. 'It's virtually impossible not to create price discovery by stopping just one stock,' he said. 'You have to stop all the associated vehicles.' BlackRock Inc. in New York and Vanguard Group Inc. said ETFs, which made up 70 percent of securities with trades on May 6 that were later canceled because of excessive declines, shouldn't be omitted from the circuit-breaker pilot. Vanguard's mutual funds had $1.42 trillion in assets as of March 31, while BlackRock's had $3.36 trillion, making it the world's biggest money manager. Schapiro has pledged to increase the scope of the pilot to 'thousands' of additional public companies. 'BlackRock has been informed by a number of large liquidity providers that when the steep decrease in U.S. equity values neared 10 percent -- a level specified as a `reference' in exchange erroneous trade rules -- the liquidity providers steeply discounted or ceased their bids for ETFs,' the firm said in a June 2 letter to the SEC. Companies 'pulled back from bidding for shares of ETFs that owned high percentages of stocks perceived as vulnerable to cancellations,' BlackRock said. The SEC said in a joint report with the Commodity Futures Trading Commission on May 18 that ETFs were the securities most affected by the May 6 plunge. ETFs, whose price is based on the value of the shares they include, declined because the price of underlying stocks decreased. The inability of market makers to hedge their trades on May 6 by buying and selling the constituent stocks may also have 'contributed to a lack of liquidity in ETF shares,' regulators found. 'If a stock within an ETF halts and the ETF continues to trade, market makers will back away from the ETF and stop making markets,' Gus Sauter, chief investment officer at Vanguard, said in an interview. This applies to firms with specific trading obligations called designated market makers on NYSE and 'unofficial people making makers' on other venues, he said. Nina Mehta reported from New York. Whitney Kisling contributed from New York. 