Using a single column of a strongly acidic cation exchange resin (Dowex 50W, X-4, 200-400 mesh, resin bed 4.0 mm diameter x 75 mm long in sodium form), noradrenaline (NA) (together with adrenaline (A)), dopamine (DA), 5-hydroxytryptamine (5-HT), and histamine (Hm), are recovered in separate eluates after a single extraction of the same tissue sample into perchloric acid. Recoveries of the amines from homogenates and filtrates were approximately the same: for NA (together with A), about 90% in a 7-ml eluate; for DA, about 95% in 3.5 ml; for 5-HT, about 75% in 7 ml; and for Hm, about 85% in 4.5 ml. The establishment and routine use of the procedure are presented in detail. Several metabolites of the amines, including 5-hydroxyindole-3-acetic acid (5-HIAA), the precursor amino acids, and 3-methoxytyramine (3-MT), can be recovered from the same Dowex 50 column by incorporating the procedures developed by Carlsson, Kehr & Lindqvist in this laboratory. All the amines and many of their metabolites can be purified successively into their eluates in a normal working day. We now have increased opportunities to study aspects of the metabolism of the biogenic amines. Thus, for the catecholamines can be monitored tyrosine + DOPA + DA + NA + A and also DA + 3-MT; and for 5-HT, the metabolic pathway tryptophan + 5-hydroxytryptophan + 5-HT + 5-HIAA. The column purification contributes considerable specificity to the overall procedure. Most compounds which are acidic; e.g. 5-HIAA, neutral or amphoteric, e.g. all the precursor amino acids, are eluted before the amines. Considerable resolution of basic compounds into separate fractions is also achieved : notably Hm is eluted before spermidine, and DA is separated from NA plus A. Thus, many compounds are prevented from interfering in fluorimetric assays which have been especially adapted for measuring these amines in the Dowex 50 eluates. Salt equivalent to at least 10 g of tissue placed on the column does not affect the recoveries of the amines. For quantitative determinations of endogenous amines, approximately 10 ng of each amine on the Dowex 50 column are sufficient, and smaller amounts can be detected. The procedure was developed particularly for measuring amines in nervous tissue, but it has been applied successfully to several peripheral tissues and body fluids.