On the 7th day of abstinence, however, a clear peak of serum testosterone appeared, reaching 145.7% of the baseline ( P < 0.01). No regular fluctuation was observed following continuous abstinence after the peak. Ejaculation is the precondition and beginning of the special periodic serum testosterone level variations, which would not occur without ejaculation. The results showed that ejaculation-caused variations were characterized by a peak on the 7th day of abstinence; and that the effective time of an ejaculation is 7 days minimum