OBJECTIVE It has been reported that massive amounts of immunoreactive endothelins (ETs) exist in human seminal plasma. However, the physiological role of ETs in seminal plasma remains to be determined. We speculated that ETs might affect sperm function. METHODS The present study was designed to investigate the effect of endothelium-1 (ET-1) on: (a) sperm motion parameters, (b) hyperactivated motility, (c) sperm-zona pellucida-binding capacity, (d) sperm-oocyte penetration capacity, (e) acrosome reaction and its prerequisite, an increase in intracellular Ca2+ concentration ([Ca2+]i), and to examine (f) the presence of binding sites for ET-1 in human sperm. Forty-six semen ejaculates from 14 fertile men were assessed under capacitating conditions after separation of the motile sperm fraction by wash and swim-up. RESULTS ET-1 (1 micron) exhibited significant stimulatory effects on sperm velocity at 30 min (P = 0.01), amplitude of lateral head displacement (ALH)max (P = 0.05), and ALHmean at 60 min (P = 0.04) in some samples (n = 10). However, these effects were not observed in experiments using a larger number of samples (n = 39). ET-1 had no effect on hyperactivated motility of sperm at 30 min to 24 h. Neither ET-3 nor IRL 1620, a selective ETB receptor agonist, affected sperm motion parameters or hyperactivated motility. ET-1 did not affect sperm-zona-binding capacity, sperm-oocyte penetration capacity, acrosome reaction, or [Ca2+]i of sperm. Specific binding sites for ET-1 were not detected on the cell surface of human sperm. CONCLUSIONS Although ET-1 is present in massive amounts in human seminal plasma and may have the capacity to alter the quality of motile sperm in some samples, a physiological role of ET-1 in the modulation of the function of mature, ejaculated sperm still remains unknown.