β-Amino proprionitrile (BAPN) at dietary concentrations of 1, 5, and 10 g BAPN/kg rat chow was administered to rats for 14-21 days following surgical constriction of the ascending aorta. Five and 10 g BAPN/kg rat chow prevented the increase in left ventricular collagen content which occurred with cardiac hypertrophy in rats following aortic constriction. In spite of this block in the increase in collagen in the ventricles, isolated trabecular muscles from hypertrophied hearts showed a decrease in maximum velocity of shortening at a preload of 0.5 g/mm2 (max V) and an increase in time to peak tension as compared with values for sham-operated animals. Max V for rats with aortic constriction was decreased 0.57 muscle length/sec as compared with sham-operated animals (P< 0.01) whereas time to peak tension was prolonged by 12 msec (P < 0.05). In rats with aortic constriction receiving 10 g BAPN/kg rat chow, max V was decreased 0.66 muscle length/sec (P < 0.05), and time to peak tension was prolonged by 21 msec (P < 0.001). Resting tension was increased to 1.70 ± 0.18 (mean ± SEM) g/mm2 as compared with shams (1.22 ± 0.10 g/mm2;P < 0.002) in cardiac hypertrophy without BAPN. However, the increase in resting tension was not seen when animals with aortic constriction received 10 g BAPN/kg rat chow [1.23 ± 0.9 g/mm2 as compared with shams, 1.15 ± 0.09 g/mm2 (not significant)]. We conclude that the decrease in maximum velocity of shortening and prolongation of time to peak tension in experimental cardiac hypertrophy occur independently of elevated collagen content, whereas elevations in resting tension appear to depend upon an increase in collagen content of these hearts.