Testes of rats, which had been injected with a single dose of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) (0.3 micrograms/kg-25 micrograms/kg body weight [BW]), were studied after 7 days using morphological and histochemical means. Light and electron microscopic examination revealed that TCDD affected testicular morphology in a dose-dependent manner. TCDD led to decreased intercellular contact, indicated by wide intercellular spaces between Sertoli cells between and Sertoli cells and neighbouring germ cells. Morphological alaterations in rat testes after TCDD administration included the sloughing off of premature spermatids into the tubular lumen and numerical increase of necrotic germ cells, in particular pachytene spermatocytes. Compared with control animals, Sertoli cells of treated rats exhibited an increased amount of lipid droplets and phagolysosomes. Vacuolization of the cytoplasm and fragmentation of the Sertoli cells occurred frequently. Examination of the different spermatogenic stages revealed that no stage was specifically susceptible to TCDD. In Leydig cells a decrease in enzyme activity of 3 beta- and 17 beta-hydroxysteroid dehydrogenases became evident by histochemical investigation. This effect on steroidogenesis was already found at a dose of 1 microgram/kg BW TCDD, whereas morphological effects were seen in the germinal epithelium for the first time at 3 micrograms/kg BW.