Lipid and fatty acid composition of testes were determined in normal rats and in rats treated with various factors affecting spermatogenesis. Cryptorchidism and cadmium chloride treatment resulted in about 30% loss of weight of testes. In the former group there was a decrease in phospholipid and an increase in concentration of cholesterol and of triglycerides. In both groups there was a decrease in concentration of palmitate and docosapentaenoate compared to normal testes. Histological examination revealed atrophy of the germinal epithelium with disappearance of spermatids and spermatozoa and of most of the spermatocytes. Treatment of rats with two anti-spermatogenic drugs resulted in no change in the concentration of docosapentaenoic acid and a reduction in the number of spermatogonia and spermatocytes. The chemical and histological changes observed are consistent with the hypothesis that the docosapentaenoate of rat testicular tissue is associated with the spermatids and spermatozoa.