The urinary production rate of melatonin has been estimated for two male subjects at approximately 30 micrograms per 24 h by measurement of the specific incorporation of deuterium label into 6-hydroxy melatonin sulphate following administration of deuterium-labelled melatonin as an isotopic tracer. No pharmacological effect due to the tracer was apparent on endogenous melatonin secretion and the production rates calculated were unaffected by either dosage or time of administration. It is, therefore, concluded that the use of deuterium-labelled melatonin coupled with quantitative gas chromatography-mass spectrometry offers a practical procedure for providing a clinically useful index of melatonin production in man.