Spermatozoa (from distal cauda epididymidis) were recovered after 10-hr incubation in the uteri of ovariectomized does primed with 17β-estradiol benzoate (EB, 1 μg/kg/day × 14; sc). Continuity between uteri and oviducts was interrupted by ligating the uterotubal junction. Average fertilization tests were 70% with sperm recovered from control capacitating animals (injected with EB only). Other capacitating animals, similarly primed received at the time of insemination a single dose of 500, 1000, 1500, 2000, or 3000μ g/kg of 20 α-hydroxypregn-4-en-3-one (20 α-OH). Using sperm incubated under such conditions, average fertilization rates in capacitation tests were 82, 62, 69, 17, and 21%, respectively. Absence of synergy was indicated by the nonsignificant difference between the average control response and the average response for dose ratios up to 1500/1. This was further confirmed by sperm penetration of the zona pellucida and fertilization in vitro using spermatozoa incubated in the uteri stimulated by EB only. However, the two highest hormone ratios tested resulted in significant (p < .01) inhibition of capicitation in the uterus. The first finding suggests that 20 α-OH does not directly participate in the hormonal stimulation of capacitation at the uterine level. Since synthesis and release of 20 α-OH by ovaries is controlled by LH, the second finding suggests a possible explanation for the inhibition of sperm capacitation in the uterus previously observed after stimulation of intact animals by high doses of exogenous gonadotropins (Soupart, 1967).