&NA;To evaluate the effects of sex hormones on heart function and biochemistry, gonadectomy (GX) was performed in postpubertal male (M) and female (F) rats and compared with sham‐operated controls (SH). The groups were MSH; MGX; MGX replaced with testosterone 3 mg/day s.c. (MGX + T), FSH, and FGX replaced with estrogen 2 mg/day (FGX + E), progesterone 0.4 mg/day (FGX + P), estrogen and progesterone (FGX + EP), or testosterone 2 mg/day (FGX + T). Body weight was decreased in MGX and was decreased further in MGX + T. Heart weight was decreased in both MGX and MGX + T. Body weights were increased in FGX and FTX + P and were increased further in FGX + T but were normal in FGX + E and FGX + EP. Heart weights were unchanged in F groups except in FGX + T, where it was increased. Cardiac performance in perfused hearts, as measured by stroke work, ejection fraction, fractional shortening and mean velocity of circumferential fiber shortening, was decreased in MGX but was slightly increased in MGX + T. Papillary muscle studies showed increases in time to peak tension and one‐half relaxation in MGX, but these were decreased in MGX + T. Isotonic shortening studies showed decreased velocity of shortening in MGX and increased velocity in MGX + T. Heart function was significantly decreased in FGX and FGX + P compared with FSH but was similar to FSH in FGX + E and FGX + EP. FGX + T had greater stroke work and ejection fraction than FSH and FGX. Parallel changes of a lesser degree and significance was observed for velocity of circumferential fiber shortening. Gonadectomy in M and F was associated with decreases both in myosin ATPase activity and the percent of V1myosin isoenzymes, and these were prevented in males by replacement with testosterone and in females by replacement with estrogen or testosterone. The results indicate important influences of sex hormones on cardiac mass, function, and biochemistry. (Circulation Research1987;61:12‐19)