But experts say such safeguards make sense only if they are applied uniformly. When the New York exchange suspended trading Thursday, sellers simply moved to other exchanges with fewer restrictions. In some cases, the supply of buyers on those exchanges already had been exhausted, causing the computerized trading programs to offer shares at lower and lower prices. Some of the resulting downward spirals ended at one penny.

“When the New York Stock Exchange went into slow motion, a system designed to stabilize trading actually backfired in practice,” said James J. Angel, a professor at Georgetown University who studies financial markets. “No exchange should have an independent circuit breaker.”

The S.E.C., which oversees the nation’s equity markets, requires a suspension in trading only in the event of a broad market collapse, defined as a drop of at least 10 percent in the Dow Jones industrial average, which is based on the share prices of 30 large American companies.

Other countries, like Germany, impose similar circuit breakers on trading in shares of any individual company that has a similar drop, but the S.E.C. has never done so. A former S.E.C. official said the possibility had been discussed in recent years, but “I don’t think there was quite the urgency to deal with it.”

The S.E.C. and the Commodity Futures Trading Commission said in a joint statement on Friday that the issue now had their attention.

“We are scrutinizing the extent to which disparate trading conventions and rules across various markets may have contributed to the spike in volatility,” the statement said. “This is inconsistent with the effective functioning of our capital markets and we will make whatever structural or other changes are needed.”

Early this year, the S.E.C. also began a broad review of equity markets, including whether computerized trading is properly regulated.