Spermatozoa from nine asthenozoospermic patients were incubated for 3 h in Krebs Phosphate Ringer (KRP), supplemented or not with 17 beta-estradiol. 17 beta-estradiol increased the mean velocity and maintained the percentage of motility during the first 2 h of incubation. Oxydative metabolism and intracellular ATP concentrations were enhanced, too, whereas glycolysis remained unchanged.