Sixteen beauer (Castor canadensis) were liue trapped, marked, and orally administered the chemosterilants 17 a-ethynylestradiol-3-cyclopentyl ether and SC-24674 ofSearle Laboratories. Treated beauer were released at the point of capture and retrapped near the end of the breeding season. Fiue treated breeding age males showed significant reduction in both testes weight (P<1J.01) and seminal uesicle weight (P<0.05) as compared to untreated males. Histological examination of testes of treated males indicated suppression of spermatogenesis and disruption of the cells of the seminiferous tubules. Fiue treated breeding age females showed significant reduction in both ouulation (P<0.005) and pregnancy (P<0.05) when compared to 25 untreated females.