In mammals, insulin stimulates granulosa cell aromatase activity and steroid production and is a regulating factor of oocyte maturation. To assess the role of insulin in human follicular and oocyte maturation, human follicular fluid was obtained 32-36 h after hCG administration at the time of oocyte recovery for in vitro fertilization. Follicular fluid insulin levels, measured by RIA, ranged from undetectable (less than 2 microU/ml) to 65.4 microU/ml. In women treated with human menopausal gonadotropin (n = 21), clomiphene citrate (n = 4), and human menopausal gonadotropin/clomiphene citrate (n = 14), follicular fluid insulin concentrations were 18.0 +/- 4.3 (+/- SE), 10.2 +/- 4.2, and 12.0 +/- 3.8 microU/ml, respectively (P = NS). Similarly, there was no significant difference in follicular fluid insulin concentrations in follicles with mature (n = 33) or immature (n = 6) oocytes (13.3 +/- 2.7 vs. 24.7 +/- 9.5 microU/ml) or in oocytes which eventually did (n = 35) or did not (n = 4) fertilize (16.4 +/- 3.0 vs. 3.2 +/- 0.8 microU/ml). Follicular fluid insulin levels (n = 30) correlated positively with follicular fluid progesterone levels (P less than 0.05), but not with follicular fluid estradiol or androstenedione levels or the estradiol to androstenedione ratio. The relationship of follicular fluid insulin and progesterone levels suggests that, as in other mammals, follicular fluid insulin may have a physiological role in follicular maturation.