Parabens are alkyl ester compounds of p-hydroxybenzoic acid widely used as preservatives in foodstuffs, cosmetics, toiletries and pharmaceuticals. These compounds are known to exert a weak estrogenic activity in estrogen receptor assays in vitro and in uterotrophic assays in vivo. In this paper, we have shown that butyl paraben had an adverse effect on the male mouse reproductive system and that it damaged the late steps of spermatogenesis in the testis. Butyl paraben was administered to 4-week-old Crj:CD-1 mice assigned to groups of eight animals, at doses of 0.01%, 0.10%, and 1.00% in the diet for 10 weeks. The average butyl paraben intake from the calculated food consumption was 14.4±3.60, 146±35.9, and 1504±357 mg/kg per day for the 0.01%, 0.10%, and 1.00% dietary butyl paraben groups, respectively. There were no treatment-related effects of butyl paraben on the liver, ventral prostates, seminal vesicles, and preputial glands (both in terms of absolute weight and relative to body weight) in any of the study groups. Both the absolute and relative weights of the epididymides were significantly higher in 1.00% group when compared with controls. A dose-dependent decrease of both round and elongated spermatid counts in stages VII-VIII seminiferous tubules was observed, and the elongated spermatid counts were significantly lower in all of the treated groups. The numbers of spermatogonia and spermatocytes did not differ from control values. The serum testosterone concentration decreased in a dose-dependent fashion and was significant at 1.00%. These data demonstrated that butyl paraben can exert an adverse effect on the male reproductive system at doses that are well below those of the accepted daily intake (ADI) in Japan.