Endothelin-like immunoreactivity specific for endothelin-1 (ET-1), endothelin-2 (ET-2) or big endothelin-1 (big ET-1) was measured, using commercially available radioimmunoassay kits, in follicular fluid collected at the time of oocyte aspiration from 36 women undergoing ovulation induction by human menopausal gonadotrophin (HMG). The relationship of ET concentrations to HMG dose, peak serum oestradiol concentration, the number and size of follicles (by ultrasound), the number of retrieved oocytes and the fertilization rate per retrieved oocyte were studied. Overall, 94% of follicular fluid samples were positive for ET-1, 92% were positive for ET-2, and 100% were positive for big ET-1. Mean ET-1, ET-2 and big ET-1 concentrations were 17.23 +/- 12.20, 32.42 +/- 14.32 and 34.55 +/- 16.34 pg/ml respectively. Endothelin-like immunoreactivity in follicular fluid samples was found in an order of ET-1 < ET-2 < big ET-1. There was a highly significant positive correlation (r = 0.8711,P = 0.001, n = 32) between follicular ET-1 and ET-2 concentrations. No significant correlation of follicular big ET-1 was established either with ET-1 or ET-2. However, big ET-1 was found to be negatively correlated with number of oocytes (P = 0.03) and number of follicles (P = 0.04). Control plasma ET-1 and follicular ET-1 were not significantly different. There was no significant correlation between ET concentrations and any of the other studied parameters. The results demonstrated that immunoreactive ET-1, ET-2 and big ET-1 exist in human follicular fluid collected at the time of oocytes retrieval for in-vitro fertilization and may be involved in the regulation of reproductive function. The clinical significance and physiological role of follicular fluid ET deserve further studies.