Estrone sulfate (E₁S) is predominantly present in the maternal circulation of pregnant animals and is excreted into the feces and urine. Hence, it is estimated that E₁S is partly present in the environment, which may be discharged from farms. The present study was undertaken to examine whether E₁S affected the endocrine function in animals. Male Japanese quails were intramuscularly treated 1mg of E₁S everyday for 4 days. Blood samples were obtained just before the treatment and at the 4 days after the onset of treatment to measure the concentrations of testosterone, E₁S and triacylglyceride. Apoptotic cell death was detected in the paraffin sections of testis. The weight of testis was significantly reduced in the E₁S treated groups compared with that of untreated group. Treatment of E₁S to the quail significantly increased plasma E₁S and triacylglyceride concentrations and significantly reduced plasma testosterone concentration. Some Sertoli cells in the E₁S treated group showed apoptotic cell death, whereas there is few apoptotic cells in the untreated group. These results indicate that E₁S has ability to induce apoptosis in the testicular cells and reduction of testicular weight and testosterone concentration, possibly due to the unconjugation of E₁S. It is also suggested that E₁S may be one of risk factors for endocrine disrupter in wild life.