Both follicle stimulating hormone (FSH) and luteinizing hormone (LH) are proposed requirements for follicular growth and steroidogenesis; however, the role of LH in primate folliculogenesis is unclear. Follicular stimulation by recombinant human FSH (n = 5) with and without recombinant LH (1:1; n = 6) following 90 days of gonadotrophin-releasing hormone (GnRH) antagonist (Antide) treatment in macaques was evaluated. Human chorionic gonadotrophin (HCG) was administered when six follicles > or = 4 mm were observed. Oocytes were aspirated 27 h later and inseminated in vitro. Chronic Antide reduced serum oestradiol and bioactive LH to concentrations observed in hypophysectomized rhesus monkeys. Multiple follicular growth required a longer interval following recombinant FSH (12 +/- 1 days) than recombinant FSH+recombinant LH (9 +/- 0.2 days), but the total number of follicles/animal did not differ between groups. The day prior to HCG, oestradiol concentrations were 4-fold less following recombinant FSH compared to recombinant FSH+recombinant LH. With recombinant FSH, more oocytes completed meiosis to metaphase II (51%) and fertilized (89 +/- 5%) relative to recombinant FSH+recombinant LH (12 and 52 +/- 11% respectively). Follicular growth and maturation in LH-deficient macaques occurred with FSH alone. Thus, LH is not required for folliculogenesis in primates. Higher fertilization rates following follicular stimulation with FSH alone suggest that the presence of LH with FSH (1:1) during the pre-ovulatory interval impairs gametogenic events in the periovulatory period.