The estimation of 5-hydroxytryptophan and its metabolites 5-hydroxytryptamine and 5-hydroxyindoleacetic acid in plasma is described. The method is based on solvent extraction and the use of a liquid cation exchange reagent to separate 5-hydroxytryptamine and 5-hydroxytryptophan. The 5-hydroxyindoles are determined fluorimetrically following derivatisation with o-phthalaldehyde in strong acid. Data on the specificity and linearity of the method are presented. The time course of plasma 5-hydroxyindoles after oral administration of 5-hydroxytryptophan is studied in two subjects as an example of the application of the method and of its possible role in monitoring the plasma levels in therapy or in provocative tests.