Total estrogens (TE), estradiol (E2), estriol (E3), and human placental lactogen (hPL) were determined by radioimmunoassay in the blood of 126 pregnant women during their 26th and 31st weeks of pregnancy and the results were studied in relation to maternal age and parity. Total estrogens and E2 were lowest among the youngest women (less than 20 years) and highest among women aged 20-24 years, whereas older women (25+ years) had, on the average, intermediate values. For E3 the pattern was qualitatively similar to that of TE and E2 but less striking, and no maternal age pattern was evident with respect to hPL. Within maternal age groups, TE and E2 were higher among women in the first, than among those in their second, full-term pregnancy; the difference was about seven percent for TE (P = 0.14) and about 14 percent for E2 (P = 0.05). No parity patterns were evident with respect to E3 and hPL. There were fairly strong correlations between the determinations of the same hormone in the same woman during the 26th and 31st weeks of pregnancy; Pearson correlation coefficients were 0.60 for TE, 0.78 for E2, 0.60 for E3, and 0.72 for hPL. Since the risk of breast cancer increases apparently monotonically with maternal age at birth, the present data are equivocal with respect to the hypothesis linking levels of pregnancy estrogens to risk of breast cancer in the offspring. However, the data are compatible with hypotheses linking excessive pregnancy-estrogen exposure to conditions more common among first-born individuals, including testicular cancer and cryptorchidism.