Abbott Laboratories said it is being investigated by the U.S. Justice Department over "sales and marketing activities" for Depakote, its third top-selling drug.

The investigation, in Virginia, includes a look at whether anti-kickback laws were violated in Medicare and Medicaid payments for the medicine, the company said today in a regulatory filing.

Depakote, used to treat bipolar mania and epilepsy, brought in $1.36 billion in 2008 sales for Abbott Park-based Abbott. That was down 13 percent from the previous year, hurt by increased generic competition.

"We are cooperating with the investigation," Abbott spokeswoman Adelle Infante said today by telephone. She declined to comment further.

Abbott shares fell 9 cents, or less than 1 percent, to $51.53 at 4 p.m. in New York Stock Exchange composite trading. The stock has lost 3.4 percent this year.