Female rats (n = 24) swam 6 h/day, 5 days/wk, for over 6 wk. Training increased both wet and dry weight of ventricles 15% above control. The active length-tension curve for trained left ventricular papillary muscles (LVPM) was elevated, with maximum tension (Tmax) at optimal length 37% higher than that of untrained LVPM. Training did not alter passive length-tension, time-to-peak tension (TPT), or half-relaxation time (1/2RT). The maximum rate of tension development (dT/dt), however, was increased 22% by training. The force-velocity curve of trained LVPM was shifted upward; both estimated Vmax and the minimum isometric load where shortening equals zero were greater than control. Exogenous isoproterenol (5.8 nM) did not affect Tmax but significantly decreased TPT and 1/2RT to the same extent for both groups of muscles. Maximum dT/dt was increased to a greater extent by isoproterenol for trained LVPM. These findings indicate hearts hypertrophied by exercise have an increased contractile potential per unit of myocardial mass.