In successive samples of human lumbar CSF, concentrations of two neurally active peptides, cholecystokinin (CCK) and neurotensin (NT), were compared with levels of homovanillic acid (HVA), the major metabolite of dopamine. Although HVA values progressively increased between the first and 20th milliliter samples, no significant change occurred in the concentration of either peptide. Thus, lumbar CSF levels of CCK and NT, unlike levels of HVA, may not closely reflect amounts of these peptides in supraspinal CSF or brain.