The authors report cerebrospinal fluid (CSF) concentrations of five monoamine metabolites before and after probenecid administration in normal subjects and patients with depression and schizophrenia. No differences were found in baseline metabolite concentrations among the three groups. CSF metabolite and CSF probenecid concentrations were significantly correlated in depressed patients for all metabolites, and there was decreased postprobenecid accumulation of homovanillic acid in the CSF of depressed patients compared with schizophrenic patients and with normal subjects. The authors propose a method for correcting for probenecid concentrations. Data from normal subjects should be of value for other investigators using the probenecid technique.