The effects of dibromochloropropane (DBCP), a pesticide, on the male rat reproductive system were examined at morphological and hormonal expression levels. Changes of Leydig cells and seminiferous tubules were examined in rats treated with one subcutaneous (s.c.) injection of DBCP at doses of 10, 50, 75 and 100 mg/kg of body weight. The Leydig cells degenerated and decreased in number in those rats with an s.c. injection of DBCP at a dose of 50 mg/kg of body weight. However, these morphological changes were not significant in those rats that were treated with four separate s.c. injections of DBCP at 10 mg/kg of body weight. The expression of luteinizing hormone receptor mRNA, which is specifically expressed in Leydig cells, was decreased significantly in the DBCP-treated rats as compared with the normal-control rats. The expression of follicle-stimulating hormone receptor mRNA was decreased to a mild degree in DBCP-treated rats. At a dose of 75 mg/kg of body weight, seminiferous tubules as well as Leydig cells were severely damaged morphologically and at hormonal receptor expression levels. Multinucleated giant cells appeared at a dose of 75 mg/kg of body weight. All the rats died at a dose of 100 mg/kg of body weight. Our data indicate that DBCP had a cytotoxic effect on the male rat reproductive system in a dose-dependent manner.