OBJECTIVE To assess the adverse effects of both in vivo and in vitro exposure to nonylphenol on the reproductive function of adult male SD rats. METHODS Adult male SD rats were sacrificed after they had received oral nonylphenol at 50, 100, 200 mg/kg for 28 days. Both indications related to their reproductive function and histopathological section of testis were analyzed. In vitro study of testicular cells from adult male rats by use of culture solution with different nonylphenol concentrations was conducted. Testosterone production in cell culture medium was examined and the ultrastructure of Leydig cell, Sertoli cell and spermatogenesis cell were observed under electron microscope. RESULTS In vivo study of the experiment animals in comparison with the controls revealed that sperm count and motility in testis of adult rats exposed to nonylphenol at doses of 100 mg/kg and above were significantly decreased, and the histopathological sections of testis showed atrophied seminiferous tubules and decreased spermatogenesis. In vitro study of testicular cell culture showed that testosterone secretion was inhibited. And electron microscopy disclosed that the endoplasmic reticulum of Leydig cell swelled, which might indicate the reduction of testosterone biosynthesis. CONCLUSIONS Nonylphenol at higher dose directly impairs the reproductive function of adult male SD rats.