The contractile properties of single rat cardiac myocytes isolated from normal and hypertrophied right ventricles have been investigated and then correlated with the isoenzyme pattern of ventricular myosin. The isoprenaline-stimulated contraction and the beta-adrenoceptor sensitivity were also investigated. Right ventricle hypertrophy was obtained by injecting monocrotaline, an alkaloid which induces severe pulmonary hypertension. The contractile parameters, namely contraction amplitude and speed of shortening, were obtained by means of an inverted microscope TV-system and edge-detection-device. Hypertrophied cells showed a significantly decreased speed of shortening and contraction amplitude when contraction was induced by maximum calcium and maximum isoprenaline. A statistically significant correlation existed between myosin alpha-chain percentage and both contraction parameters. Isoprenaline sensitivity expressed in terms of ED50, p kappa a and Hill coefficient were similar in the control and monocrotaline treated animals. Moreover no correlation between the degree of ventricular hypertrophy and ED50 existed. These results suggest that, in this animal model, the depressed contractile function which characterizes the hypertrophic myocardium depends on changes in isomyosin pattern while beta-adrenoceptor desensitization does not occur.