PRL concentrations were measured in cord plasma obtained at hysterotomy from 26 rhesus monkey fetuses between 111--170 days gestational age (GA). Mean PRL concentrations increased significantly from 23.7 +/- 10.1 (X +/- SE) ng/ml at 121--130 days GA to 126.9 +/- 16.9 ng/ml at 161--170 days GA. A similar significant increase in PRL with age also was observed in samples obtained from 16 fetuses chronically catheterized in utero between 130--155 days GA. Mean PRL levels were 34 +/- 3.2 ng/ml at 131--140 days GA and rose to 82 +/- 9.7 at 150--155 days GA. No difference in PRL concentrations was found between cord blood samples and fetal peripheral blood samples at the ages studied. Maternal PRL levels did not change in samples obtained from chronically catheterized, chair-restrained mothers between 130--155 days GA. A tendency toward an increase in maternal PRL with advancing gestational age was observed in samples collected after hysterotomy. These data indicate that the fetal rhesus monkey demonstrates an increase in plasma PRL similar to that in the human, suggesting a possible physiological role for this hormone in the primate fetus late in gestation.