Exchanges passed two recent tests of their ability to handle anexplosion of trading. They face a third and potentially greaterchallenge today, when the Federal Reserve decides whether to raise interest rates for thefirst time in nine years.

On Aug. 24, as anxiety engulfed markets worldwide and promptedone of the worst days for stocks since the aftermath of the 2008financial crisis, no exchanges reported malfunctions. U.S. stockexchanges weathered a surge in volume to a four-year high of 14.1billion shares. Last year, a wild session for Treasuries and otherassets prompted no obvious malfunctions on Oct. 15, when 11.9billion U.S. shares traded.

Technical errors at prominent financial markets in recentyears have eroded public confidence and prompted regulators todemand they do better. With the Fed's decision likely to unleashanother tsunami of volume, exchanges face another big test.