alpha-chlorohydrin (ACH) is a known male reproductive toxicant and produces antifertility in rats. The present experiments were performed to determine the relationship between sperm motions and reproductive function, and to further examine the possible mechanism for antifertility. ACH was administered to male rats for 9 days at 1, 3 and 10 mg/kg/day. The males were mated with untreated females and their reproductive status was determined. All mated males failed to impregnate females at 10 mg/kg/day. Low pregnancy rate associated with a decreased implant number was seen at 3 mg/kg/day. When sperm motions were analyzed using the CellSoft computer-assisted sperm analyzer, percentage of motile sperm, curvilinear velocity (VCL) and amplitude of lateral head displacement (ALH) were reduced at 10 mg/kg/day. At 3 mg/kg/day, VCL and ALH were reduced but the percentage of motile sperm was comparable to that of controls. In order to examine a possible mechanism for the effect of ACH on fertility, the number of sperm reaching the oviducts of mated females and the number of fertilized eggs was evaluated. Half of the females mated with ACH-treated males at 3 mg/kg/day had very low sperm numbers in the oviducts. At 10 mg/kg/day, all the mated females had a very low sperm number. The percent of fertilized eggs in the oviducts of mated females was decreased in a dose-dependent manner. These findings suggest that the effect of ACH on fertility was directly related to decreased VCL and ALH as well as percentage of motile sperm, and by the mechanism in which the sperm number reaching the oviducts after mating was reduced, so the reduction resulted in only a rare chance to fertilize.