The purpose of this study was to determine the changes in sex hormone level in men after ejaculation. The serum testosterone concentrations of 28 male volunteers were investigated daily during abstinence period after ejaculation. We found that fluctuations of testosterone levels from day 2 to day 5 of abstinence were minimal. On day 7 of abstinence, a peak of serum testosterone appeared, reaching 145.7% of the baseline (P<0.01). After the peak, no regular fluctuation was observed. Ejaculation was the premise and beginning of the 7 days' periodic phenomenon. If there was no ejaculation, there was no periodical changes in serum testosterone level. These results indicate that the periodic change in serum testosterone level is caused by ejaculation.