Eight mongrel dogs exercised for 8 weeks by treadmill running at 20 per cent incline 20 to 25 minutes twice daily, 4-5 days/week. Another eight dogs which were kept in the cages for a similar period served as controls. The exercise program was effective in inducing myocardial hypertrophy since the ratio left ventricular weight/body weight was significantly (P less than 0.001) higher in the trained dogs (5.04 g/kg) than in the sedentary animals (3.83 g/kg). In morphine-chloralose anesthesia the dogs were studied by left heart catherization and cineangiography at spontaneous heart rate (run I), at paced heart rate (run II), at paced heart rate following cardiac autonomic nervous blockade by bilateral vagotomy and the administration of propranolol (run III) and during acute pressure loading with methoxzmine at constant heart rate (run IV). Intergroup comparison yielded no significant difference in any hemodynamic or volumetric parameter throughout the entire study. However, with intragroup comparisons between run III and run IV a less significant increase in left ventricular end-diastolic pressure (from 5 to 15 mm Hg; P less than 0.05) was observed in the trained animals than in the control dogs (from 6 to 25 mm Hg; P less than 0.001). Left ventricular end-diastolic volume increased significantly only in the control dogs during acute pressure loading. Mean aortic pressure and left ventricular peak dP/dt increased to a similar extent in both groups. Since in the trained dogs the left ventricle encroaches less on the Frank-Starling mechanism than in normal animals for overcoming an acute pressure burden it is concluded that the development of hypertrophy concomitant with chronic exercise represents an adaptive mechanism with evidence of beneficial consequences for the intrinsic contractile function of the myocardium.