An assay has been developed for brain tryptophan using reverse-phase liquid chromatography with electrochemical detection. The method simultaneously assays dopamine (DA) and its metabolites, dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), as well as 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA). The method does not require elution from ion exchange resins. After deproteinization and centrifugation samples are injected directly onto the chromatographic column. It was found that small changes in mobile phase pH markedly influenced the retention time of tryptophan while elution of the indoleamines and catecholamines did not change. The assay of these endogenous compounds in a single injection proved not expedient but inexpensive. Values obtained using alumina and ion exchange resins yielded comparable values.