Recent measurements of circulating inhibin A and inhibin B concentrations indicate that inhibin B may play an important role in the selection of dominant follicles. The concentrations of inhibin A, inhibin B and activin A were measured in the follicular fluids of 61 individual follicles (4.8-20 mm in diameter) from 47 regularly cycling women using specific two-site enzyme-linked immunosorbent assays. The microenvironment of each follicle was characterized by measuring follicular fluid androstenedione and oestradiol concentrations. The mean activin A concentrations were < 8 ng/ml for follicles of all sizes (4-17 mm). Inhibin A concentrations were < 1 ng/ml in follicles < 6 mm, and progressively increased to concentrations > 50 ng/ml in follicles > or = 13 mm. Follicles with androstenedione/oestradiol ratios < or = 4 had higher concentrations of inhibin A than follicles with androstenedione/oestradiol ratios > 4. Inhibin B concentrations were higher than inhibin A concentrations in all follicles, increasing from 19.2 +/- 8.3 ng/ml in 4 mm follicles to 409 +/- 9.6 ng/ml in 13 mm follicles and then declining to 275 +/- 47 ng/ml in 17 mm follicles. These results support the hypothesis that inhibin B may play a more important paracrine role in developing follicles and a greater regulatory role with respect to follicle stimulating hormone (FSH) secretion than inhibin A.