The authors examined the effects of the immunosuppressive drug cyclosporine (CsA) on the male reproductive system in prepubertal rats. Twenty-one-day-old rats were subcutaneously injected with either cremaphorsaline vehicle or CsA (1 and 2 mg/kg/d). The animals were treated until they were 66 days old. Cyclosporine did not affect the weights of the body or testis but decreased the weights of all sex accessory organs. Quantitative analysis of the tubules in stage VII of spermatogenesis revealed a decline in the cell counts of pachytene spermatocytes and step VII spermatids. Testicular and epididymal sperm counts and motility were decreased by 50% and fertility by 60%. Cyclosporine lowered serum testosterone despite an elevation of LH, indicating that the drug directly inhibited testosterone synthesis. Serum creatinine levels were normal in the treated animals, precluding renal failure as the cause for this impairment. Intratesticular concentrations of pregnenolone and 17-hydroxy progesterone were significantly elevated, while those of progesterone, androstenedione, and testosterone were markedly reduced. Determination of steroidogenic enzyme activities indicated that the administration of CsA inhibited the activity of delta 5-3B-hydroxy steroid dehydrogenase-delta 5-4 isomerase (3 beta-HSD). These results clearly indicate that CsA in the doses used is harmful to the male reproductive function in prepubertal rats.