The main focus of this study is the optimal administration period concerning toxic effects on male fertility in rats. To assess functional and morphological changes induced in the testis by nitrazepam, male rats were administered the drug at doses of 0, 20, 40 or 80 mg/kg during pre-mating periods of 2, 4 or 9 weeks and then the 2 weeks of mating. At the end of the administration period the animals were sacrificed and sperm number, motility, abnormalities and histopathological changes in the testis were examined. Decreases in testis weight, epididymis weight, number of sperm in the testis and sperm motility were observed in the 40 and 80 mg/kg sections of the 2, 4 and 9 week pre-mating treated groups. Mating with untreated females revealed no adverse effects on copulation rate in any group; however, a remarkable decrease in pregnancy rate was noted in the 80 mg/kg section of the 2, 4 and 9 week treated groups. On histological examination, various degrees of localized necrosis in the seminiferous epithelium and Leydig cell hyperplasia were observed in the testis. No clear changes were observed in the 20 mg/kg section of the 2 week pre-mating administration group, but at the 4 week time point, necrosis of spermatogenic cells began to appear. The primary morphological event was evident in spermatocytes with necrosis of the cytoplasm observed from 4 weeks after administration of nitrazepam, although sperm motility and sperm head counts were unaffected. From these findings, examination of sperm characteristics and histopathological changes in the testis are important parameters for evaluation of drugs inducing testicular damage. We conclude that a 4 week administration period is sufficient to detect effects of nitrazepam on male fertility.