We examined tryptophan and serotonin (5-hydroxytryptamine) levels in the blood after consumption of alcohol. Forty-five minutes after drinking, whole blood serotonin concentration was significantly reduced, whereas no changes were observed in tryptophan level. The diurnal rhythm of 5-HT in subjects who the day before had drunk alcohol was quite different from the control group, but very similar to that of patients with depression. The results strongly suggest that the mechanism of depression after alcohol drinking may be related to serotonin.