Nashville-based pharmacy benefits manager Caremark Rx, the largest publicly traded company in Middle Tennessee as ranked by revenues, is merging with drugstore chain CVS in a deal that removes Caremark from the list of locally based public companies.

CVS will exchange 1.67 shares of its stock, which was trading at $28.94 early this afternoon, for each share of Caremark, which opened today at $53.65 on news that talks were underway. Thus the deal appears to value Caremark at between $20 billion and $21 billion, or $48 to $49 per share. Caremark's stock fell beneath $47 on the news, and at 1:40 the price stood at $47.71, down 3.09 percent on yesterday's close.