Follicle development was induced in 41 women with follicle-stimulating hormone (FSH) and human menopausal gonadotropin. Blood samples were drawn and follicular fluids (FF) were aspirated when two or more follicles attained diameters of 15 to 17 mm. Levels of estradiol (E2), progesterone (P), FSH, and luteinizing hormone (LH) were determined in samples by radioimmunoassay, and relationships between the measured parameters in antral fluids and in serum were examined by least-squares linear regression analysis. Levels of LH in serum correlated with LH and FSH levels in FF (P less than 0.005). Concentrations of FSH in serum were positively related to levels of LH, FSH, E2, and total protein in FF (P less than 0.005). E2 levels in serum were predictive of E2 levels in FF only, and levels of P in serum were directly correlated with P levels in FF (P less than 0.05). With respect to the peculiarly broad range of predictions that could be drawn from the FSH content of serum, peripheral FSH provided a better predictive index of the chemical composition of antral fluid than did the level of any other single hormone measured in serum.