Concentrations of follicle-stimulating hormone (FSH) and human chorionic gonadotropin (HCG) were measured in the sera of 45 women during normal pregnancy and in a separate group of 9 women during delivery and at daily intervals in the puerperium. Cord sera were also studied. In addition, serum FSH and luteinizing hormone (LH) concentrations were measured in a group of 15 women at 6 weeks post partum. Serum FSH concentrations averaged 0.39 mU/ml (NIH-FSH-S1) and did not vary throughout pregnancy; the values were in the high normal range for adult nonpregnant premenopausal women. In the assay system the FSH material detected in pregnancy serum reacted in a fashion identical with that of the pituitary FSH standard. Highly purified HCG and human placental lactogen were also assayed and did not cross-react significantly in the FSH assay. There was no correlation between serum FSH and HCG concentrations. HCG and FSH concentrations were higher in maternal than in cord sera. The mean HCG and FSH maternal: cord ratios were 9.3 and 1.6, respectively. Serum HCG concentrations decreased abruptly after delivery, with an estimated maximal initial halflife of 6.6 hr. Serum FSH decreased only slightly in the first 2 days post partum and increased to near the initial level by post-partum day 5. Serum FSH and LH concentrations were within the normal range (adult female premenopausal) in both lactating and nonlactating subjects at 6 weeks post partum.