Tryptophan, an essential amino acid, is the substrate for the biosynthesis of melatonin. Therefore, the hypothesis of a possible effect of dietetic tryptophan supplementation on melatonin synthesis deserves attention. It is assumed that tryptophan is synthesized by ruminal microbes and no deficiency is expected in ruminants. Cows and heifers were treated with a tryptophan supplement during a week. The concentrations of melatonin were determined in plasma and milk using an ELISA kit in the morning and evening. On the seventh day of treatment, melatonin concentration was significantly higher in the milk of trypto-phan-supplemented cows than in the control cows (Kollmannet al., 2008). L-tryptophan supplementation to aged ring doves reversed the disturbances of circadian rhythm related to ageing (Garau et al., 2006) and its effect in the European sea bass has been also demonstrated (Herrero et al., 2007). Up to the date of writing this review, there are no referenced reports concerning the effect of tryptophan supplementation on melatonin release in humans. Nevertheless, these studies show the need for an assay to determine naturally occurring tryptophan when aiming to evaluate the effect of a food on melatonin-endogenous concentration. Table 2 presents the tryptophan content for a number of food items, some of which have been tested for melatonin content.