We have shown that advancing the increase in maternal serum estrogen levels from the second to the first third of baboon pregnancy suppressed extravillous cytotrophoblast (EVT) spiral artery invasion. Since vascular endothelial growth factor (VEGF) promotes EVT invasion, the present study determined whether EVT VEGF expression is altered by prematurely elevating estrogen in early pregnancy. Placental basal plate was obtained on day 60 of gestation (term is 184 days) from baboons treated daily on days 25–59 with estradiol (0.35 mg/day sc), which increased maternal peripheral serum estradiol levels 3-fold above normal. Overall percentage of uterine arteries (25 to over 100 µm in diameter) invaded by EVT assessed by image analysis in untreated baboons (29.11 ± 5.78%) was decreased 4.5-fold (P<0.001) by prematurely elevating estrogen (6.55 ± 1.83%). VEGF mRNA levels in EVT isolated by laser capture microdissection from the anchoring villi of untreated baboons (6.77 ± 2.20) were decreased approximately 5-fold (P<0.05, ANOVA) by estradiol (1.37 ± 0.29). Uterine vein serum levels of the truncated sFLT1 receptor, which controls VEGF bioavailability, in untreated baboons (403 ± 37 pg/ml) were increased 3-fold (P<0.01) by estrogen treatment (1,127 ± 197 pg/ml). Thus, placental EVT expression of VEGF mRNA was decreased and serum sFLT1 levels increased in baboons in which EVT invasion of the uterine spiral arteries was suppressed by advancing the rise in estrogen from the second to the first third of pregnancy. We suggest that VEGF mediates the decline in EVT vessel invasion induced by estrogen in early primate pregnancy.