The quantities of estrone and estradiol were determined by radioimmunoassay in maternal and fetal plasma of the rhesus monkey from day 59 to 163 of gestation. A two way analysis of variance of data classified according to fetal sex and 3 pooled gestational ages for each hormone and for mother or fetus (4 analyses) revealed significant elevations in fetal estradiol and maternal estrone concentrations with age. All other comparisons were not significant by these analyses. The concentrations of estradiol were greater in maternal than in fetal plasma [769 +/- 64 (SE) pg/ml, N = 63 VS 57 +/- 6 (SE) pg/ml, N = 77, P less than 0.01] by a t test. Estrone, on the other hand, was similar in mother and fetus [265 +/- 30 (SE) pg/ml, N = 60 vs 318 +/- 37 (SE) pg/ml, N = 73, P greater than 0.05]. No sex differences in the concentrations of these hormones were observed except in the fetus after 150 days of gestation. At this time plasma from female fetuses contained significantly more estradiol than plasma from male fetuses [118 +/- 20 (SE) pg/ml, N = 7 vs 61 +/- 10 (SE) pg/ml, N = 19, P less than 0.01]. Except for estradiol in female fetuses, the concentrations of estrogen were significantly higher in the umbilical vein than in the umbilical artery, an indication that the placenta is a major source of fetal estrogen in this species. Estrone and estradiol were significantly correlated in both the fetal and maternal circulation, r = 0.58, P less than 0.001 and r = 0.39, P less than 0.01 respectively. The results provide quantitative data about the estrogen miliue in which the monkey fetus develops and suggest mechanisms for controlling fetal estrogen in this species.