Powdered Solanum lycocarpum fruit is commonly used to treat diabetes, but apparently no studies have been conducted to evaluate potential adverse side effects. In the present paper the toxic effect of S. lycocarpum was evaluated in adult male Wistar rats and Swiss mice. The administration of an aqueous extract prepared using a powder obtained from the S. lycocarpum fruit at two different dose levels (60 mg/15 ml and 120 mg/15 ml distilled water for rats and 30 mg/15 ml and 60 mg/15 ml distilled water for mice, twice daily for 5 days in each case) did not produce body weight variations in either species although a significant weight change was observed in some organs. Significant weight loss was observed only in the ventral prostate of mice receiving the high dose treatment. These results suggest a toxic effect of S. lycocarpum on the male reproductive system of the Swiss mouse, with possible antiandrogenic activity, but there was no apparent antifertility activity in rats at the doses given.