In order to evaluate for future physiological and pharmacological studies the extent to which orally administered melatonin is found in human serum and saliva and excreted into urine we measured serum, saliva and urine concentrations of melatonin by radioimmunoassay after oral administration of 100 mg melatonin. Elevated melatonin concentrations were observed with peak values of 435 nmol/l in serum and 241 nmol/l in saliva at 60 min. Elimination was monophasic following first-order kinetics. The half-lives for serum and saliva melatonin were 41 and 38 min, respectively. The results suggest that melatonin is passively secreted into saliva which reflects closely the changes in serum melatonin. Saliva sampling is thus useful in studies on peripheral melatonin both in physiological and experimental conditions. Urinary excretion of melatonin was 0.01 % of the amount of melatonin ingested. In high-performance liquid chromatography urine extracts were found to contain also a minor unknown immunoreactive component which we suggest to be some unknown metabolite of melatonin.