Male Sprague–Dawley (SD) rats were exposed to 4-t-nonylphenol (NP) by gavage at dosages of 0, 125 and 250 mg/kg/day for 50 days.Organ weights of liver, kidney, testis and epididymis were measured. Sperm number in the head of epididymis was counted. Several hormonesincluding testosterone, luteinizing hormone and follicle stimulating hormone were measured. Testicular sections were observed by light andelectron microscopy. Terminal dideoxynucleotidyl transferase dUTP nick end labeling assay was performed to probe the apoptotic cells inseminiferous tubules. When rats were treated with nonylphenol at 250 mg/kg/day, the absolute and relative weight of epididymis decreaseddramatically, while the relative weights of kidney and liver increased by 14 and 22%, respectively. In addition, the sperm density of the headof epididymis and the testosterone level descended at 250 mg/kg/day. The levels of luteinizing hormone and follicle stimulating hormoneincreased in both nonylphenol treated groups. Pathological changes were detected by microscopy and the transferase dUTP nick end labeling(TUNEL) assay showed that the number of apoptotic cells in testes increased with nonylphenol in a dose-dependent manner.