U.S. markets are tumbling after the open, with the Dow off more than 230 points, or 1.8 percent at the open. 
The Nasdaq is 2.4 percent lower while the S&P 500 is down 1.5 percent.
A sell-off in technology and energy firms is weighing on the S&P 500, with only nine of the 500 components in the green. More than 370, or three-quarters, of the S&P 500 is negative.
The trade followed an impressive list of merger announcements, including Cnooc's purchase of Nexen for $15.1 billion, NRG Energy's takeover of GenOn Energy for $1.7 billion, and Genesee & Wyoming purchase of RailAmerica for $1.4 billion.
Below, starting trade in the Dow.
Dow
Bloomberg