Recent experimental studies have suggested that the initial nonstimulated stage of dynamic cardiomyoplasty acutely impairs ventricular function. Those investigations were performed on normal hearts and primarily examined diastolic alterations as a result of the passive muscle wrap. The purpose of this study was to assess the acute systolic and diastolic effects of a nonstimulated muscle wrap in chronic heart failure induced by rapid ventricular pacing in canines. Pressure-volume analysis of ventricular function based on conductance catheter volume and micromanometer pressure data was used. Each animal was studied before rapid pacing, before cardiomyoplasty, and immediately after wrap. By the end of the pacing period and before wrap, left ventricular dysfunction developed in all dogs, manifested by significant deterioration of both systolic and diastolic indices of ventricular function, as well as progressive increases in left ventricular volumes. However, no further deterioration with load insensitive indices of systolic or diastolic indicators of ventricular function was found as a result of the passive muscle wrap. These results suggest that the cardiomyoplasty procedure can be safely performed on failing hearts without prohibitive acute impairment of ventricular function.