Sprague-Dawley male rats were administered nefiracetam orally at daily doses of 500 and 1500 mg/kg/day for 4 or 9 weeks. Although the copulation index was not affected by nefiracetam treatment, the fertility index was extremely low in the 1500 mg/kg/day group for both treatment periods. This high dose group consistently exhibited decreased testicular weights. Epididymal and prostate weights were also reduced in the 1500 mg/kg/day group after both 4- and 9-week treatments and in the 500 mg/kg/day group after the 9-week treatment. Severe degenerative changes such as degeneration of germ cells, loss of germ cells and atrophy of seminiferous tubules were observed in all fats of the 1500 mg/kg/day groups after both 4 and 9 weeks of treatment. Retention of spermatids in stage IX, X and XI seminiferous tubules was also noted after the 4- and 9-week treatments at 500 mg/kg/day. The testicular sperm head counts were markedly decreased following the 4- and 9-week treatments at 1500 mg/kg/day, and mildly reduced after the 4-weeks treatment at 500 mg/kg/day. From these results it is concluded that histopathological examination and the testicular sperm head count method are highly useful for detecting testicular toxicity and that testicular lesions caused by nefiracetam can be detected after 4 weeks of exposure.