Adult male Sprague-Dawley rats were exposed to tap water containing 0, 8,571, 17,143, or 34,286 ppm cypermethrin for 12 weeks. Based on water consumption per animal per day the rats received 13.15, 18.93, and 39.66 mg cypermethrin, respectively. Fertility was significantly reduced in male rats ingesting cypermethrin at a concentration of 13.15 and 18.93 mg in that the number of females impregnated by them was significantly reduced. The number of implantation sites was significantly reduced in females mated with males that had ingested cypermethrin at a concentration of 39.66 mg. A significant reduction in the number of viable fetuses was observed in females impregnated by the exposed males at all three doses of cypermethrin. The body weight gain was significantly lower in the treated males. Ingestion of cypermethrin at a concentration of 18.93 or 39.66 mg per day resulted in a significant increase in the weights of testes and seminal vesicles. Preputial gland weights were increased at all three concentrations of cypermethrin. Epididymal and testicular sperm counts as well as daily sperm production were significantly decreased in exposed males. The serum levels of testosterone, follicle-stimulating hormone and luteinizing hormone were significantly reduced in males exposed to 39.66 mg per day. Ingestion of cypermethrin at 18.93 and 39.66 mg/animal/day also resulted in a significant decrease in the perimeter and number of cell layers of the seminiferous tubules. The testes of treated animals were infiltrated with congested blood vessels with marked hemorrhage and a significant accumulation of connective tissue surrounding the seminiferous tubules, which contained a large number of immature spermatids. These results clearly demonstrate the adverse effects of cypermethrin pesticide on fertility and reproduction in male rats.