The present study was conducted to determine whether estrogen regulates the P-450 cholesterol side-chain cleavage (P-450scc) enzyme component of the progesterone biosynthetic pathway in the placenta during the second half of baboon pregnancy. Placental estrogen formation was suppressed by removing the fetus, i.e. fetectomy, and thus fetal adrenal C19-steroid estrogen precursors, on day 100 of baboon gestation (term = 184 days). P-450scc activity and messenger ribonucleic acid (mRNA) levels were then determined in placentas obtained on day 160 after fetectomy alone and after fetectomy and s.c. administration of the estrogen precursor and rostenedione or estradiol benzoate. Placentas were maintained in situ after fetectomy, and placental villi were comprised of syncytiotrophoblasts that seemed morphologically normal, based on their histology and immunocytochemical expression of pregnancy-specific-beta 1-glycoprotein. In untreated baboons, peripheral serum estradiol increased with advancing gestation, and mean (+/-SE) concentrations were 1.22 +/- 0.05 ng/ml on days 101-160 of gestation. After fetectomy serum estradiol concentrations decreased to 24% (P < 0.01) of normal. Androstenedione or estradiol administration after fetectomy increased serum estradiol levels to values that were 57% (P < 0.01) of, or 90% (P < 0.001) greater than intact controls, respectively,. Placental P-450scc specific activity, determined on a mitochondrial-enriched fraction of villous tissue, was 281.1 +/- 15.0 pmol pregnenolone plus progesterone formed per mg mitochondrial protein in untreated control baboons. Fetectomy resulted in a 52% decrease (P < 0.001) in placental P-450scc activity. Administration of androstenedione or estradiol after fetectomy increased P-450scc activity to values that were not significantly different from control. P-450scc mRNA levels were quantified by competitive RT-PCR. P-450scc mRNA levels in placental villous tissue of fetectomized baboons was 38% lower (P < 0.01) than that in the intact controls (110.9 +/- 5.9 attomoles/microgram RNA). The administration of androstenedione after fetectomy restored P-450scc mRNA to a level that was not different from the untreated controls. The results of this study show that there was close association between the levels of estrogen and the specific activity of and the mRNA levels for placental P-450scc in the second half of baboon pregnancy. Therefore, we propose that the P-450scc enzyme that catalyzes the conversion of substrate cholesterol to pregnenolone is regulated, for the most part, by estrogen in the primate placenta.