Merck (NYSE: MRK) and sanofi-aventis (NYSE: SNY) terminated plans to combine their animal health businesses, citing increasing complexity of implementing the proposed transaction.

The companies mutually determined that ending their plan is in the best interests of both companies and their respective shareholders.

The companies are discontinuing their agreement primarily because of the increasing complexity of implementing the proposed transaction, both in terms of the nature and extent of the anticipated divestitures and the length of time necessary for the worldwide regulatory review process, Merck and sanofi-aventis said.

The joint venture announced on March 2010 was to have combined Sanofi's Merial animal health unit, the maker of the Frontline tick fighter, with Merck's Intervet/Schering-Plough business. Merck bought Schering-Plough in November 2009.

Between them, Merial and Intervet had $5.5 billion in sales in 2010.

Following the termination, both Merial and Intervet/Schering-Plough will continue to operate independently, the companies said.

ADR's of Sanofi closed Monday's trading at $34.00, while shares of Merck ended at $32.39.