To study the influence of physical training on myocardial contractile function, 6 cats were trained by having them swim 45 min/day 5 days/wk, for 20 wk, and 10 cats were maintained as sedentary controls. Right ventricular papillary muscles were studied in vitro (isometric contractions), and adenylate cyclase activity was determined from ventricular muscle. The heart weight-to-body weight ratio was 7.5% greater in the trained cats than in the sedentary cats, but the difference was not statistically significant. Myocardial contractile function during control conditions was similar in both groups of muscles. Likewise, the enhancement of function with paired stimulation and with increasing frequency of stimulation was not significantly different in the 2 groups. Enhancement of contractile function was significantly greater in trained than in sedentary cats during stimulation by 10-8 M (P < 0.05) and 10-7 M (P < 0.10) isoproterenol. The enhancement of myocardial adenylate cyclase activity was greater in trained than in sedentary cats during stimulation by 10-5 M (P < 0.05) and 10-4 M (P < 0.10) isoproterenol or norepinephrine. Physical training does not alter the intrinsic contractile function of cardiac muscle, nor does it alter the contractile responses to paired stimulation or increasing stimulation frequency. Training may improve the catecholamine-induced enhancement of both myocardial contractile function and adenylate cyclase activity.