A method was developed for measuring endogenous acid metabolites of dopamine (dihydroxyphenylacetic acid, DOPAC; homovanillic acid, HVA) and of 5-hydroxytryptamine (5-hydroxyindoleacetic acid, 5HIAA) in serial samples of cerebroventricular perfusates from unanesthetized, freely-moving rats by the use of high performance liquid chromatography with electrochemical detection. Systemic administration of probenecid increased while pargyline reduced the efflux of all of the acid metabolites. Intraperitoneal injection of L-tryptophan increased the perfusate content of 5HIAA, decreased HVA and was without effect on DOPAC. Injections of haloperidol increased the efflux of DOPAC and HVA but did not alter the efflux of 5HIAA. Administration of apomorphine decreased the concentration of all three compounds. These drug-induced changes are generally consistent with those reported to occur in brain tissues. The results indicate that this method can be employed to monitor the activity of dopamine and 5-hydroxytryptamine neurons in unanesthetized, freely-moving rats.