Glutathione reductase (GR) recycles oxidized glutathione (GSSG) by converting it to the reduced form (GSH) using an NADPH as the electron source. The function of GR in the male genital tract of the rat was examined by measuring its enzymatic activity and examining the gene expression and localization of the protein. Levels of GR activity, the protein, and the corresponding mRNA were the highest in epididymis among testes, vas deferens, seminal vesicle, and prostate gland. The localization of GR, as evidenced by immunohistochemical techniques, reveals that it exists at high levels in the epithelia of the genital tract. In testis, GR is mainly localized in Sertoli cells. The enzymatic activity and protein expression of GR in primary cultured testicular cells confirmed its predominant expression in Sertoli cells. Intracellular GSH levels, expressed as mol per mg protein, was higher in spermatogenic cells than in Sertoli cells. As a result of these findings, the effects of buthionine sulfoximine (BSO), an inhibitor for GSH synthesis, and 1,3-bis(2-chlorethyl)-1-nitrosourea (BCNU), an inhibitor for GR, on cultured testicular cells were examined. Sertoli cells were prone to die as the result of BCNU, but not BSO treatment, although intracellular levels of GSH declined more severely with BSO treatment. Spermatogenic cells were less sensitive to these agents than Sertoli cells, which indicates that the contribution of these enzymes is less significant in spermatogenic cells. The results herein suggest that the GR system in Sertoli cells is involved in the supplementation of GSH to spermatogenic cells in which high levels of cysteine are required for protamine synthesis. In turn, the genital tract, the epithelia of which are rich in GR, functions in an antioxidative manner to protect sulfhydryl groups and unsaturated fatty acids in spermatozoa from oxidation during the maturation process and storage.