The effect of prolactin on the glycolytic metabolism of spermatozoa from married infertile men whose semen had spermatozoal concentrations of less than 40 million/ml was studied. In a respirometry system, spermatozoa were incubated with L-U-14C-glucose as substrate. Glucose utilization and production of 14C-lactate by the spermatozoa in the presence of 100 ng of ovine prolactin were greater than in the absence of the hormone. The production of pyruvate was 2.5 times less than the production of lactate wehn spermatozoa were incubated in the absence of prolactin and 1.6 times less in the presence of prolactin. These results indicate that, in spite of having an increased glycolytic metabolism, spermatozoa from oligospermic men are are able to show additional glucose utilization in the presence of prolactin.