The present studies were conducted to gain a better understanding of the effects of pregnenolone (P₅), human chorionic gonadotropin (hCG) and 3' 5', cyclic adenosine monophosphate ( cAMP) on porcine placental and endometrial production of progesterone (P₄), testosterone (T) and estrone (E₁) at 30, 60 and 90 days of gestation. Duplicate 300 mg samples of placenta, endometrium or both (co-incubation) were incubated in medium199 containing either no P₅, P₅, P₅ + hCG or P₅ + cAMP for either zero (control), .5, 1 or 2 h. The first study compared P₄ and E₁ production with or without addition of P₅. At d 30, 60 and 90, respectively, P₄ production (ng/g) increased significantly in the presence (vs absence) of P₅ in the incubation medium of placental (13.2 vs 7.5, 73.9 vs 42.7, 137.4 vs 113.5, respectively) coincubation (14.5 vs 10.0, 33.6 vs 22.3, 77.9 vs 49.4, respectively) and endometrial (16.0 vs 13.3, 23.0 vs 16.0, 17.1 vs 6.7, respectively) tissue. Presence of P₅ increased E₁ production in d 60 (1.3 vs .7 ng/g) and d 90 (51.7 vs 34.6 ng/g) placental tissue and d 90 endometrial tissue (9.8 vs 8.0 ng/g). In a second study, P₅ + cAMP increased (vs P₅ alone) P₄ in placental tissue at d 30 (11.6 vs 8.7 ng/g) and coincubation tissue at d 90 (103.7 vs 75.3 ng/g). Cyclic AMP stimulated increased P₄ synthesis ( vs P₅ alone), throughout the incubation period in d 60 and d 90 tissue. E₁ production by endometrial tissue at d 30 (4.1 vs 2.9 ng/g), and placental tissue at d 60 ( 1. 2 vs . 9 ng/g). Presence of hCG in the incubation medium had no overall effect on either P₄ or E₁ accumulation. Only trace amounts of T were detected in either study, suggesting rapid aromatization of C₁₉ steroids to estrogens. / M.S.