The method is an extension of the chromatographic separation procedure for 5-HIAA and 5-HTP published by Lindqvist (1971). Brain tissue is extracted with perchloric acid. After adjustment of the pH to 2 the extract is passed through a Dowex 50W, X-4 column. 5-HIAA is eluted with 60% aqueous methanol, DOPA and tyrosine with 0.1 M sodium citrate buffer pH 4.5 and 5-HTP and tryptophan with 0.1 M sodium phosphate buffer pH 6.5. If the above procedure is combined with that published by Atack and Magnusson (1970) 9 different compounds can be estimated after a single column procedure. DOPA is assayed fluorimetrically using a modification of the trihydroxy-indole (THI) assay for noradrenaline (Bertler et al., 1958). The level of DOPA found in a normal rat brain was below the sensitivity of the method (10 ng/g). After inhibition of the aromatic amino acid decarboxylase a compound indistinguishable from authentic l-DOPA accumulated in the rat brain.