The determination of platelet serotonin (5-HT) and plasma tryptophan concentrations is useful in the diagnosis, investigation of etiologies, and treatment of psychiatric disorders. To determine the usual circadian variations in platelet 5-HT and free and total tryptophan concentrations, we measured these variables during 24 h at 1-h intervals and every 30 min from 2000 to 0800 in seven clinically healthy young men with an HPLC method. No common circadian rhythm for platelet serotonin concentrations was observed in our subjects; however, there was a distinct rhythm for both free and total plasma tryptophan: Concentrations were maximal in the afternoon and minimal during the night.