Superfusion with a cyclopropane fatty acid, cis-9,10-methylenehexadecanoic acid (10-300 mu M), reduced the contractility of papillary muscle isolated from guinea pigs in a dose-dependent manner. cis-9,10-Methylenehexadecanoic acid also inhibited the Mg2+-ATPase activity of guinea pig papillary myocardium by about 40% at 400 mu M. Since cis-9,10-methylenehexadecanoic acid 4 mu M inhibited the K+-EDTA-ATPase activity inherent in myosin's catalytic activity by about 25%, the fatty acid was thought to interact with the catalytic center of the myosin molecule.