The Dow Jones industrial average closed down 36.52 at 6,268.34, after giving up a gain of 50 points earlier today.
Decliners outnumbered advancers by nearly 2-to-1 on the New York Stock Exchange, with 887 up, 1,673 down and 787 unchanged.
NYSE volume totaled 447.58 million shares vs. 454.46 million in the previous session.
The Standard & Poor's 500-stock index fell 7.65 to 720.99, and the NYSE's composite index fell 3.22 to 380.85. The Nasdaq composite index fell 23.82 to 1,261.09, and the American Stock Exchange's index declined 3.49 to 572.57.
Most of the Dow's gains earlier were built on Boeing shares, which rose sharply after the company this weekend announced it would buy rival aircraft and defense maker McDonnell Douglas in a stock swap valued at $13.3 billion.
The Boeing announcement kept stocks aloft throughout the morning even as bonds gave ground following an economic report this morning. The long bond was down 9-16 point, yielding 6.61 percent.
Bonds fell after the Federal Reserve said America's industrial output shot up 0.9 percent in November, the biggest gain in nine months. That was almost twice what was widely expected.
The New York Stock Exchange imposed a trading collar on upside action in the morning, when the Dow was up 50 points, and a collar on the downside in the afternoon.