The present study was undertaken to evaluate the effect of evening primrose oil (EPO) on the male sexual functions. EPO (daily 0.5 ml/mouse) was orally intubated for 28 consecutive days to experimental ICR mice, and same vol. of vehicle to control mice. On the 14th and 28th experimental day, the testis weight, number of complete intromissions and mating, serum testosterone and cGMP levels, prostaglandin leveIs of penile corpus cavernosum smooth muscle cells, and NO-productive activity of endothelial cells were determined. The weight of body and testis, the number of complete intromissions during the 3hour period were somewhat increased in EPO treated mice than those of control. The number of sperm-positive females and testosterone level in serum were increased in experimental groups. The serum cGMP level was significantly increased but the NO production of ionomycin-stimulated HUVEC cells was not affected when EPO was added into cultures. These results suggest that oral administration of EPO enhanced the sexual functions of male mice, and EPO could be developed as a tonic improving sexual functions.