We investigated the effects of 30-480 ng/L 17alpha-ethinylestradiol (EE(2)) on the sperm motility and fertility of Japanese medaka (Oryzias latipes). Sperm motility was examined by computer-assisted image analysis. In male medaka, the velocity of sperm was found to have increased after 3 weeks of exposure at 60-480 ng/L. This result suggests that higher sperm velocities depleted sperm energy reserves more rapidly and shortened the time for which sperm were viable to fertilize eggs. In a separate experiment that studied whether EE(2) exposure of males affects the fertilization rate or hatchability, sexually mature male medaka were exposed for 3 weeks and subsequently evaluated for their reproductive ability after pairing with unexposed females for 7 days. Exposure of males to EE(2) exerted a potent inhibitory effect on a reproduction parameter (fertilization rate x hatchability), and the highest inhibition was observed at 60 ng/L. The results offer toxicological data for the assessment of EE(2 )exposure in medaka and suggest that short-term exposure to EE(2) might reduce sperm function and fertility in adult male medaka.