The two-cell theory predicts that follicular steroidogenesis requires the coordinate actions of both FSH and LH; however, the role of LH in follicular growth is less clear. The present study was designed to investigate the relative importance of LH and FSH in follicular growth and steroidogenesis. Cynomolgus monkeys were treated with a GnRH antagonist (antide; 3 mg/kg.day) for 20 days beginning in the midluteal phase of the menstrual cycle. After 10 days of antide administration, monkeys were injected with recombinant human FSH (rhFSH; 10 IU; n = 3), human menopausal gonadotropin (hMG; 10 IU; n = 3), or FSH plus 0.5 IU LH (n = 3) twice daily for 10 days. rhFSH stimulated multiple follicular development; however, peak serum estradiol levels were only 943 +/- 195 pmol/L. In contrast, monkeys treated with the same dose of hMG had significantly higher (P < 0.05) peak estradiol levels (6013 +/- 1322 pmol/L). The addition of 0.5 IU LH to the rhFSH treatment resulted in serum estradiol levels similar to those in monkeys treated with rhFSH only. Importantly, no differences in follicle number or size were evident among these treatment groups. Follicular fluid estradiol levels were consistent with serum levels (rhFSH, 187 +/- 11 nmol/L; hMG, 1531 +/- 173 nmol/L). Even larger proportional differences in follicular fluid androstenedione (rhFSH 13.6 +/- 1.4 nmol/L; hMG, 307 +/- 97.7 nmol/L) levels were found. The results in this LH-deficient primate model suggest that FSH alone is capable of stimulating ovarian follicular growth; however, the resulting follicles manifest minimal estradiol production, probably due to deficiencies in the LH-induced precursors to estradiol.