Effects of fetectomy on peripheral plasma levels of estrogens and progesterone were studied at 10 weeks (3 monkeys) and 16 weeks gestation (4 monkeys). Fetectomy was followed by a decrease in maternal peripheral plasma estradiol 17-β (E 2) at a time when E 2 levels remained elevated in intact pregnant monkeys. Estrone (E 1) levels, initially low at fetectomy (10 weeks), were maintained at similar low levels in contrast to elevated concentrations observed in normal animals during the final 30 days of pregnancy. In the absence of the fetus, progesterone (P) levels were similar to those of the normal pregnancies. After removal or delivery of the placenta, P levels decreased rapidly. Maternal hypophysectomy in one animal (10 weeks) produced a transient decrease in E 2 followed by recovery to control levels by 16 weeks. Subsequent fetectomy (16 weeks) was followed by a precipitous decline in maternal E 2 levels. In conclusion, results indicate: a fetal origin, possibly from adrenal precursors, for the major contribution of E 2 during the last 3 months and E 1 during the last month of gestation; and placental production of most of the P found in peripheral plasma of pregnant rhesus monkeys.