The aim of the study was to investigate the effect of propranolol upon the activity of proteases in rat myocardium subjected to aortic stenosis. In acute heart hypertrophy induced by aortic stenosis, the activity of all three proteases in the myocardium does not change significantly. Propranolol in the concentration of 10(-6) to 2 X 10(-4) M inhibited proteolytic activity dependent on neutral proteases. The degree of inhibition increased simultaneously with increasing concentrations of propranolol. Propranolol in a low concentration (10(-6)-10(-5) M) also inhibited proteolytic activity dependent on alkaline and acidic proteases, but in higher concentration (10(-4)-2 X 10(-4) M) stimulated proteolytic activity of these enzymes 2-3 times.