The objective of this investigation was to compare intrinsic cardiac function in male and female rats. The first series of experiments (Series I) compared hearts from age-matched males and females at 18 weeks of age. Since body and heart weight were 65 and 51 per cent greater respectively in male rats in this study, a second series of experiments was performed (Series II) where heart mass was matched in males and females by studying males at 10 weeks of age and females at 18 weeks of age. Cardiac function was studied in an isolated working heart apparatus at similar heart rates and at controlled levels of end-diastolic pressure and aortic diastolic pressure (ADP). In Series I coronary flow and enddiastolic volume (both per g LV) were the same in male and female hearts, while stroke work (per g LV), ejection fraction and fractional shortening were significantly greater in male hearts but only as the ADP was increased. In Series II all of these measurements were significantly greater in male hearts at all loading conditions. The magnitude of the differences between male and female hearts was greater in Series II than in Series I which may have been due to a Frank-Starling effect in male hearts from Series II. The results indicate that intrinsic cardiac function is moderately greater in male rats compared to female rats. Studies employing similar size male and female hearts suggest that differences in heart mass may have accounted for the differential response to ADP in Series I.