NASHVILLE - Express Scripts Inc. said yesterday that it had begun its $25 billion offer to buy Caremark Rx Inc., a rival pharmacy-benefits manager.

Caremark has rejected the bid in favor of a deal with drugstore operator CVS Corp.

Express Scripts is offering Caremark stockholders $29.25 in cash and 0.426 shares of Express Scripts stock for each share of Caremark stock they hold.

Based on closing stock prices on Friday, the Express Scripts offer has a value of $56.87 a share, or about $25 billion, and provides Caremark stockholders with a 7 percent premium over the current value of the CVS proposal.

CVS, of Woonsocket, R.I., the nation's largest operator of drugstores, said Nov. 1 that it planned to acquire Caremark for about $21.2 billion in stock.

Express Scripts, of Maryland Heights, Mo., launched its bid, then worth about $26 billion, for Caremark on Dec. 18. Caremark rejected the offer early this month, but Express Scripts has persisted, making an offer directly to shareholders yesterday.

"While we would prefer to meet with the Caremark board and management to negotiate a transaction between Express Scripts and Caremark, we are taking this action in light of the Caremark board's rejection of, and refusal to even discuss, our superior proposal," Express Scripts said in a news release.

Caremark spokeswoman Nina Devlin said yesterday that Express Scripts' tender offer to shareholders was "nothing new and nothing unexpected" and that the company remained committed to its deal with CVS.

Caremark, of Nashville, has argued that it was unlikely that regulators would approve a deal combining two of the nation's three biggest pharmacy-benefit providers.

Whether or not the Caremark and CVS merger goes through may depend on a shareholder vote expected in mid-March or a Delaware judge who is considering a Caremark shareholder lawsuit claiming that the deal benefits Caremark executives more than shareholders.

Last week, Express Scripts attorneys asked a Delaware court to void a $675 million breakup fee, arguing that it unlawfully locks Caremark into the deal with CVS.

Caremark shares fell 58 cents, to $56.25, and CVS shares fell 15 cents, to $31.79, on the New York Stock Exchange. Express Scripts shares rose 88 cents, to $65.71, on the Nasdaq Stock Market.

Gregg Haddad, an analyst with First Analysis Securities Corp., of Chicago, said the Express Scripts tender offer "is just another tactic on Express Scripts' part to reach the conclusion it wants."

"I think it makes sense," he said. "Time is of the essence from Express Scripts' perspective. CVS obviously has the upper hand here. So it's really incumbent upon Express Scripts to keep its proposal in front of" Caremark shareholders.