The ventricular myocardium was studied in A/J mice and in Sprague-Dawley rats. In male mice, the ventricles were slightly larger and the specific activities of the lysosomal hydrolases, beta-glucuronidase, hexosaminidase, beta-galactosidase, and arylsulphatase, and the inner mitochondrial enzyme cytochrome c oxidase were substantially higher than in female mice. Orchiectomy abolished this sex difference. Testosterone administration induced myocardial hypertrophy and accretion of RNA and protein without altering the DNA, and substantial increases in the activities of the lysosomal hydrolases and cytochrome c oxidase. However, the mitochondrial membrane enzyme monoamine oxidase was unaffected by sex, orchiectomy, and testosterone administration. Heart lysosomes from male mice showed a smaller structure-linked latency of the lysosomal enzymes and a greater fragility of the lysosomal membrane to osmotic and mechanical stress than those from female mice. This sex difference was also abolished by orchiectomy and restored by testosterone replacement. Similar sex differences were observed in the rat with respect to heart size, acid hydrolase activities, and lysosomal enzyme latency and membrane stability. These findings indicate that endogenous androgens regulate myocardial cell growth, the activity of enzymes associated with lysosomes and the inner mitochondrial membrane, and some physiochemical properties of lysosomes.