We recently showed that the number of primordial follicles was reduced by 50% in ovaries of near term fetal baboons deprived of estrogen in utero and restored to normal in animals supplemented with estrogen. Oocytes are avascular and rely on surrounding granulosa cells for nutrients, a process facilitated by microvilli on the oocyte surface. However, our understanding of oocyte microvillus development in the primate fetal ovary is incomplete. Thus, we determined whether estrogen regulates formation of oocyte microvilli in utero. Fetal ovaries were obtained on day 165 of gestation (term=d 184) from baboons untreated (n = 3) or treated on days 100-165 with aromatase inhibitor CGS 20267 (estrogen suppressed by 95%; n = 5) or CGS 20267 and estradiol (n = 4). Follicles with intact (homogeneous cytoplasm) or non-intact (cytoplasm vacuolated) oocytes were quantified and the number/height of oocyte microvilli determined by electron microscopy. In untreated baboons, the mean (±SE) number of follicles/0.08 mm² with an intact oocyte (11.5 ± 0.5) was decreased (P < 0.05) by 70% in fetal ovaries of estrogen-suppressed baboons (3.4 ± 0.2) and restored (P < 0.05) by CGS 20267 and estradiol (11.2 ± 1.2). In estrogen-deprived fetuses, the number of microvilli/intact oocyte (23 ± 3) was 56% lower (P < 0.01) than normal (52 ± 5) and restored by CGS 20267 and estrogen (62 ± 4). Moreover, in intact oocytes of estrogen-suppressed baboons, height (nm) of microvilli (105 ± 11) was 54% - 62% lower (P < 0.01) than in intact oocytes of fetal ovaries of baboons untreated (228 ± 13) or estrogen-treated (274 ± 17). In estrogen-replete baboons, the number of microvilli in intact oocytes was 2-fold greater (P < 0.01) than in non-intact oocytes. However, in estrogen-deprived baboons, no microvilli were detected in non-intact oocytes and the number of microvilli in intact oocytes was similar to that in non-intact oocytes of untreated fetuses. We conclude that development of microvilli in oocytes of primordial follicles in the primate fetal ovary is regulated by estrogen. Collectively, these results and those of our previous studies indicate that estrogen regulates fetal ovarian folliculogenesis and development of follicles with oocytes comprised of microvilli critical for nutrient uptake and presumably long term survival.