SYNTHESIS in the brain of the putative transmitter 5-hydroxytryptamine (5HT) is influenced by the concentration of tryptophan; the rate limiting step in 5HT synthesis is the hydroxylation of tryptophan1 by tryptophan hydroxylase, which is normally unsaturated with substrate2. Often when 5HT turnover in the rat brain increases—as indicated by higher concentrations of its metabolite 5-hydroxyindoleacetic acid (5HIAA)—there is a corresponding increase in brain tryptophan. This occurs when the rats are given various drugs known to increase brain 5HT synthesis3, deprived of food4,5, or immobilized5. Therefore the control of brain tryptophan concentration is clearly of some importance.