Melatonin in rat pineal, plasma, and urine: 24-hour rhythmicity and effect of chlorpromazine.

The concentrations of melatonin in rat pineals and in samples of rat plasma and urine were measured by bioassay or radioimmunoassay. Melatonin excretion during the daily dark period (0.98 +/- 0.07 ng/12 h, by bioassay; 1.40 +/- 0.08 ng/12 h, by radioimmunoassay) was much greater than during the light period (0.18 +/- 0.04 ng/12 h, by bioassay; 0.40 +/- 0.04 ng/12 h by radioimmunoassay). (The radioimmunoassay is not completely specific when applied to materials extracted from utine.) Pineal glands and plasmas obtained from animals killed during the daily dark period also contained much more melatonin than samples from animals killed during the light period. Pretreatment with chlorpromazine slowed the disappearance of exogenous melatonin from rat plasma and markedly raised the concentrations of endogenous melatonin in both the pineal gland and the plasma.