Details are presented of an improved selected ion monitoring assay for the major biogenic amine metabolites and probenecid in human lumbar cerebrospinal fluid (CSF). The metabolites and probenecid are simultaneously extracted with ethyl acetate from an acidified aqueous phase, and are simultaneously converted to pentafluoropropionyl esters by reaction with pentafluoropropionic anhydride and pentafluoropropanol. The esters of the metabolites are analyzed following a single injection of the derivatized sample onto the gas chromatographic column, while the ester of probenecid is analyzed following a separate injection onto the gas chromatographic column. Quantitation is achieved using for internal standards dueterated analogues of the metabolites and a chemical analogue of probenecid. Data are presented on the concentration of free and conjugated forms of the metabolites in lumbar CSF taken from healthy volunteers.