Prolactin (PRL) is a pituitary hormone that plays important roles in mammalian reproductive physiology, specially lactation. The regulation of PRL secretion shows important species differences. To study PRL regulation in a subhuman primate, the Cebus apella, we developed an heterologous radioimmunoassay using an antibody against rhesus PRL (anti-m5PRL) and a Cebus apella pituitary extract as PRL standard. The assay has a sensitivity that allows measurements of cebus PRL in small amounts of Cebus apella plasma obtained from animals in different physiological conditions. Plasma cebus PRL concentrations (+/-SEM) varied in different reproductive stages. PRL concentration in adult Cebus apella females that have regular menstrual cycles (161.6 +/- 15.0 mIU/ml) was similar to that found in adult (100.3 +/- 7.6 mIU/ml) and prepuberal males (101.2 +/- 3.9 mIU/ml). PRL concentration was higher in pregnant (677.8 +/- 11.8 mIU/ml) and in nursing (625.0 +/- 47.0 mIU/ml) Cebus apella females than in 15-d post-partum non-nursing (369.0 +/- 19.0 mIU/ml) and cycling females. PRL concentration in Cebus apella newborns (719.0 +/- 49.2 mIU/ ml) was similar to that found in pregnant and nursing females, and higher than in the other females as well as adult and prepuberal males. These differences in PRL concentration in different physiological conditions are similar to that observed in humans and other primates. A PRL response to thyrotropin releasing hormone (TRH) was demonstrated in 2 nursing Cebus apella females, similar to the response found in nursing woman and rhesus. Altogether, the data presented support the proposal that the assay developed to measure PRL in Cebus apella is an adequate tool to study the physiology of PRL in this species.