Endothelin levels were measured by means of [125I]endothelin-l,2,3 radioimmunoassay in the follicular fluids of 22 mature ovarian follicles of 11 patients, obtained during ovum pickup. In all but 1 of the follicles, endothelin levels were detectable and ranged between 0.2 and 3.6 pg/ml (mean 1.47 ± 0.99). In this preliminary study, no correlation was found between follicular endothelin levels and oocyte fertilization or the hormonal ovarian response during the follicular phase.