The objective of this study was to evaluate endothelial vs. steroidogenic cell proliferation throughout the lifespan of the primate corpus luteum during the menstrual cycle and simulated early pregnancy (CG treatment). Tissues were collected from rhesus monkeys (Macaca mulatta; n = 3/day) on days 3-4, 7, 10, 12, and 14 of the of the luteal phase and at menses during spontaneous menstrual cycles and after 1, 3, 6, or 9 days of hCG treatment beginning on day 9 of the luteal phase. Corpora lutea were snap-frozen in mounting medium for immunocytochemical and histochemical evaluation. The labeling index (percentage of positive to total nuclei) for Ki-67 antigen, a cell proliferation marker, was determined in conjunction with cell-specific markers. Immunolocalization of platelet/endothelial cell adhesion molecule-1 and von Willebrand factor in addition to histochemical staining for the Ulex europaeus agglutinin-1 (i.e. lectin)-binding site were used to identify endothelial cells. Histochemical detection of 3 beta-hydroxysteroid dehydrogenase activity was used to identify steroidogenic cells. Progesterone secretion was high on days 3-10 of the luteal phase and then declined progressively (P < 0.05) on days 12 and 14 and at menses; luteal weight followed a similar pattern, declining 2 days (i.e. day 14) after progesterone secretion. In contrast, after hCG treatment, luteal progesterone production increased (P < 0.05) 3-fold, and luteal weight was maintained. The cell proliferation index was greatest (44.5 +/- 1.9%) on days 3-4 of the luteal phase and remained high on days 7 and 10 (34.6 +/- 0.3% and 27.1 +/- 3.4%), but this was followed by a sharp decline on day 12 (9.6 +/- 2.3%), which was sustained on day 14 and at menses. After 1 day of hCG treatment, cell proliferation was less than that observed on the equivalent day of the luteal phase (day 10), but thereafter, it was similar on days 3, 6, and 9 of simulated early pregnancy to those in the late luteal phase of the menstrual cycle (i.e. day 12 to menses). Dual label immunocytochemistry indicated that more than 85% of cells staining positively for the Ki-67 antigen costained for platelet/endothelial cell adhesion molecule-1. No cells staining positively for both 3 beta-hydroxysteroid dehydrogenase activity and the Ki-67 antigen were noted. Thus, the level of cell proliferation within the primate corpus luteum varies during the luteal lifespan in the menstrual cycle, and endothelial cells comprised the vast majority of proliferative cells, whereas steroidogenically active cells were not proliferating. Further, the elevated progesterone secretion and sustained luteal weight that occurred during CG exposure simulating early pregnancy were not associated with an increase or maintenance of cellular proliferation.