PURPOSE: Vitis vinifera is a species of Vitis that is native to the Mediterranean region, central Europe, and southwestern Asia, and has been used as a drug in traditional medicine. Traditional medicinal plants have been used for medical purposes with increasing effectiveness. It is important to identify drugs that inhibit spermatogenesis. Therefore, the present study aimed to investigate the effect of grape juice (GJ) on serum levels of inhibin B and sperm count in normal male rats. MATERIALS AND METHODS: Thirty-five adult male rats were randomly divided into five groups, each containing seven rats. Rats in the control group received 1 mL of normal saline over the course of the study. The experimental groups received GJ (100, 200, 400, and 1,600 mg/kg, orally, for 35 days consecutively). At the end of the treatment period, fertility indices were measured, including body weight difference, sex organ weight, sperm motility and count, epididymal sperm reserve, daily sperm production (DSP), and serum inhibin B levels. RESULTS: We found that GJ reduces body weight difference, was associated with decreased sperm motility and count in all treatment groups (p< or =0.05 and p< or =0.001, respectively). Moreover, DSP was significantly decreased in all treatment groups compared to the control group (p< or =0.05), except in the group receiving 100 mg/kg of GJ. Inhibin B levels were significantly decreased in all treatment groups (p< or =0.05). CONCLUSIONS: The results of our study suggest that GJ in all doses, but especially in higher doses, may decrease fertility in male rats.