To determine whether gastrin and cholecystokinin (CCK), recently found in the central nervous sytem, were present in cerebrospinal fluid (CSF), we studied human specimens by sensitive and specific radioimmunoassays for the two related polypeptide hormones. The concentration of gastrin in cerebrospinal fluid from 10 neurologically normal persons ranged from 1.5 to 8.0 pM (mean 3.4 pM), whereas the concentration of CCK ranged from 4 to 55 pM (mean 14 pM). The molecular heterogeneity of gastrin and CCK in CSF was determined by gel chromatography of concentrated fluid monitored by 3 gastrin radioimmunoassays specific for different sequences of gastrin17 and 3 CCK radioimmunoassays specific for different sequences of CCK33. Chromatography revealed that gastrin was present in molecular forms corresponding to gastrin34 ('big gastrin') and gastrin17. CCK was present in molecular forms corresponding to the COOH-terminal octapeptide amide of CCK33 and a fragment corresponding to sequence 25-29 of CCK33. Also, a peptide corresponding to COOH-terminal tetrapeptide amide common to both gastrin and CCK was found. The results indicate that true gastrin as well as CCK are present in CSF, and that both hormones display a molecular heterogeneity similar to that found in extracts of brain tissue.