To examine the effect of carboplatin on testicular function, carboplatin was administered to mature male Sprague-Dawley rats. Four weeks after carboplatin treatment, the serum level of follicle-stimulating hormone (FSH) significantly increased in animals treated with 30 mg kg-1 carboplatin (p < 0.02). After 8 weeks of administration, the sperm count and motility in the cauda epididymis decreased in the rats treated with 30 mg kg-1 carboplatin (p < 0.005 and p < 0.05, respectively). The percentage of a-type tubules and the diameter of seminiferous tubules were also significantly reduced in the group treated with 30 mg kg-1 carboplatin (p < 0.02 and p < 0.01, respectively). The serum levels of luteinizing hormone and testosterone were not changed, although the serum level of FSH increased in the animals treated with 20 and 30 mg kg-1 carboplatin (each, p < 0.005). All parameters returned to normal values 12 weeks after the treatment. In conclusion, carboplatin impairs spermatogenesis by possibly damaging the spermatogonia and Sertoli cells but does not affect the Leydig cell function.