Pharmacy benefits manager Express Scripts Inc. said Tuesday it is in the middle of a contract dispute with WellPoint Inc., one of the biggest health insurers in the United States.

Express Scripts said WellPoint has raised the possibility of filing a lawsuit, but it said the companies are negotiating, and Express Scripts believes the companies can resolve the dispute. It said WellPoint is disputing the implementation of some terms of the contract and "certain operational matters associated with Express Scripts' performance" under those terms. The contract between the companies went into effect on Dec. 1, 2009.

WellPoint, based in Indianapolis, operates Blue Cross Blue Shield plans in 14 states and provides health insurance for more than 34 million people, which makes it the nation's largest health insurer based on membership.

St. Louis-based Express Scripts has spent most of this year embroiled in a fight with Walgreen Co., the largest U.S. drugstore chain. A contract between Express Scripts and Walgreen expires at the end of 2011, and the companies have not been able to agree how much Express Scripts should pay Walgreen to fill prescriptions. If they do not agree to a new contract, most people whose prescription drugs plans are managed by Express Scripts won't be able to fill their prescriptions at Walgreen stores.

Express Scripts disclosed the dispute in a form filed with the Securities and Exchange Commission.

Shares of Express Scripts fell 2 percent, to $44.40 each, Tuesday and slipped another 3.6 percent, or $1.60, to $42.80, in after-hours trading. WellPoint stock declined 75 cents, to $64.64, and was unchanged in aftermarket trading.