The effect of prostaglandin F2α (PCF2α) and of prostaglandin E2 (PGE2) on the ovarianΔ 5-3β-hydroxysteroid dehydrogenase (Δ5-3β-OH-SDH) and 20α-hydroxysteroid dehydrogenase activities (20α-OH-SDH) was studied in pregnant rats at various stages of gestation. The prostaglandins were infused intravenously over 6-7 hr and the rats were sacrificed at intervals of 24-96 hr after the start of the infusion, in order to study the ovarian enzymic activity by histochemical methods. While the Δ5-3β-OH-SHD activity was not significantly influenced by either PGF2α or PGE2 at any stage of gestation, both compounds had a marked effect on the ovarian 20α-OH-SDH activity. PGF2α was effective at a ten times lower dose than PGE2. After infusion on Days 9-12, strong 20α-OH-SDH activity appeared within 24 hr in all corpora lutea of pregnancy which normally are devoid of any activity at that stage of gestation. This 20α-OH-SDH activity remained clearly detectable for at least 4 days following the infusion, while in early gestation, on Days 4-6, PGF2α had only a slight and transient effect on 20α-OH-SDH which was no longer demonstrable 3 days after the infusion. In the last half of gestation, PGF2α always induced this activity in the corpora lutea, but of variable intensity according to the stage of gestation, being moderately strong at 14 days, weak at 16 days and strong at 18 and 20 days of gestation. The appearance and intensity of this activity was well correlated with the effect on gestation. PGF2α terminated pregnancy only at periods when strong 20α-OH-SDH activity appeared in the ovaries following the infusion. Concomitant treatment with progesterone or with daily injections of LH prevented the interruption of pregnancy and the appearance of 20α-OH-SDH activity. Prolactin had no effect on the PGF2α-induced changes in corpus luteum.