Parabens are a group of compounds widely used as preservatives in foodstuffs, cosmetics, toiletries and pharmaceuticals. These compounds are known to exert a weak estrogenic activity, with butylparaben showing the most potent activity among methyl-, ethyl- and propyl esters in in vitro recombinant yeast assay and in in vivo uterotrophic assay. To account for potential reproductive effects in male animals, butylparaben was administered to 3-week-old Wistar rats divided in groups of eight subjects, at doses of 0.00%, 0.01%, 0.10% and 1.00% with the animal's diet. After 8 weeks, the rats were killed by decapitation and the weights of the testes, epididymides, prostates, seminal vesicles and preputial glands were recorded. The absolute and relative weights of epididymides were decreased in a dose-dependent manner and the decrease was statistically significant at 0.10% and above. The cauda epididymal sperm reserve of all treated groups was significantly decreased. The sperm count of the group receiving the highest dose was 58.2% of control values. The daily sperm production (DSP) in the testis was also significantly lower in all treated groups when compared to controls. Serum testosterone concentration was lowered dose-dependently and was significant at 0.1% or more. The daily intake of butylparaben that caused these disruptions is similar to the lower level of acceptable daily intake (ADI) for parabens in the European Community (EC) and in Japan. The results of the present experiments show for the first time that exposure of a postweaning mammal to butylparaben had an adverse effect on the secretion of testosterone and in the functions of the male reproductive system.