Using a newly characterized anti-melatonin serum it has been possible to establish human serum melatonin concentrations at short time intervals during 24 h. A clear circadian rhythm with peak values during the dark phase was demonstrated in both men and women. Values (pg/ml; mean ± SE) were as follows: females 02.00 h: 130 ± 7, 18.00 h: 45 ± 5; males 02.00 h: 140 ± 11, 18.00 h: 70 ± 5. The estimation (pg/ml; mean ± SE) of melatonin in human serum (males: 63 ± 22 and females: 100 ± 45) and cerebrospinal fluid simultaneously taken has shown that melatonin is lower in cerebrospinal fluid (males: 59 ± 33 and females: 57 ± 28). Blanks are not subtracted.