Human mixed saliva extracted with chloroform was found to contain immunoreactive melatonin which eluted in HPLC as synthetic melatonin. There was a clear diurnal rhythm in saliva melatonin with low day and high night levels. A similar rhythm was observed in serum. The high night levels in saliva were lower than the corresponding levels in serum. The calculated secretion rate of melatonin into saliva was less than 0.5-0.9 pmol X h-1 during the day and 1.0-2.4 pmol X h-1 during the night. Saliva sampling can be used as a simple and non-invasive way of studying melatonin secretion in human body.