The relation of single cell electrical activity to generation of force was determined for myocardium during the development of hypertrophy induced by pressure loading. Transmembrane potentials and isometric active force at optimal length (Po) were monitored in isolated right ventricular papillary muscles and trabeculae carneae after inducing chronic right ventricular outflow tract obstruction. Cats subjected to partial pulmonary artery occlusion or to sham operation were studied 1, 3, 7, 10, 21, and > 90 days after surgery. Right ventricular muscles were stimulated at 30/min at optimal length in Tyrode's solution (36°C). Muscles from sham-operated and 1 day pressure-loaded hearts had electrical and mechanical properties similar to normal muscles. Right ventricular peak systolic and end-diastolic pressures were increased for 1 and 3 day pressure-loaded hearts; the percent of right ventricular water was increased for 3, 7, and 10 day pressure-loaded hearts. Muscles from 3 day pressure-loaded ventricles had significantly decreased Po, maximum rate of force development, time to peak force, and duration of contraction compared with normal muscles, and their action potential plateaus originated at more negative potentials (mean 24.7 mv for 11 muscles). The alteration in action potential configuration was less obvious in muscles studied 7 and 10 days after partial pulmonary artery occlusion and did not occur 21 and > 90 days after pressure loading; however, Poand maximum rate of force development remained decreased, although time to peak force and duration of contraction returned to normal for preparations from 7, 10, 21, and > 90 day pressure-loaded ventricles.