To determine the effects of ejaculation on serum PSA, we measured serum levels just before masturbation and 24 hours and 5 days later in a study group (n=25) aged between 23 and 25 years. In the study group, 16 cases showed a decrease (mean 22.37%, range 10-50%) in serum PSA levels 24 hours after ejaculation, while 6 had higher levels (mean 38.33%, range 21-67%) and 3 had no changes. No relation was found between seminal plasma levels or total amounts expelled of this marker and the difference in serum levels due to ejaculation. In the control group free of ejaculation in the same period determinations of serum PSA levels revealed no significant changes between days 0, 1 and 5. As compared with the control group, the changes in the study group were found to be statistically insignificant. These results may indicate that ejaculation has an insignificant effect on serum PSA levels.