Although it has been indicated that many neurotoxicants also cause reproductive toxicity, the reproductive toxicity of megadoses of pyridoxine, which is a neurotoxicant, has not been studied. In this paper, we studied the effects of megadoses of pyridoxine on male reproductive organs. Pyridoxine hydrochloride, 125 mg/kg, 250 mg/kg, 500 mg/kg or 1000 mg/kg, daily, was intraperitoneally injected into Wistar male rats 5 days a week for 2 or 6 weeks, and its effects on the male reproductive organs were investigated. After 2 weeks of administration, absolute weights of the testis in the 500 and 1000 mg/kg epididymis in all the exposed groups and prostate gland in the 1000 mg/kg group decreased, and mature spermatid counts in the testis decreased in the 1000 mg/kg group. After 6 weeks administration, the absolute and relative weights of the testis, epididymis, prostate gland and seminal vesicle decreased in the 500 mg/kg and 1000 mg/kg groups, and mature spermatid counts in the testis and sperm counts in the epididymis decreased in these groups. Among the marker enzymes of the testicular cells, LDH-X activity decreased, and beta-glucuronidase activity, cytochrome P-450 content and cytochrome b5 content increased in the 1000 mg/kg group. Plasma testosterone concentration did not significantly alter in all the exposed groups. From these results, it was concluded that megadoses of pyridoxine affected the spermatogenesis and decreased reproductive organ weights in the rat.