In a speech closely watched by investors, Bernanke said the Fed will consider a range of steps at its Sept. 20-21 meeting, but he offered no specific plans. The Dow Jones industrial average fell 100 points soon after Bernanke began speaking.
"The implications are that the Fed is going to act, but the market is disappointed because he was a little short on details," said Scott Brown, chief economist at Raymond James.
Concerns about the economy have pushed stocks lower each month since April. Many traders now say the stock market is pricing in the assumption that the economy is in a recession, meaning limited job growth and a weaker corporate profits.
The Dow average lost 119.05 points, or 1.04 percent, to 11,295.81. The Standard & Poor's 500-index fell 12.72, or 1.06 percent, to 1,185.90. The Nasdaq composite shed 19.80, or 0.78 percent, to 2,529.14. Each index posted gains earlier in the day.
Cisco Systems Inc. led the 30 Dow stocks with a 2.58 percent gain.
Not all of the economic news Thursday was negative. American exports of cars, airplanes and other goods reached an all-time high in July, the Commerce Department reported. -- AP