A sensitive ion-pair reversed phase liquid chromatographic method for determining tryptophan, 5-hydroxytryptophan, 5-hydroxytryptamine (serotonin) and 5-hydroxyindole acetic acid has been developed. To separate these indoles in cerebrospinal fluid both octadecyl- and phenyl-bonded columns were tested. The effect of an ion-pair reagent (n-heptyl sulphonate) and pH of the mobile phase on the separation was examined. With the method described the concentrations of the four indoles can be determined within 45 min without sample pretreatment. The sensitivity obtained with electrochemical detection is 0.45 mumol/l for tryptophan and 2-4 nmol/l for 5-hydroxyindoles in human cerebrospinal fluid samples.