CSF somatostatin and homovanillic acid (HVA) were measured in 14 schizophrenic patients while they were drug-free and during chronic fluphenazine treatment. CSF somatostatin was significantly reduced and CSF HVA was significantly elevated (p less than 0.002) during fluphenazine treatment. There was a trend toward correlation between CSF somatostatin and CSF HVA in the 14 schizophrenic patients when drug-free (r = 0.49, p less than 0.07) and fluphenazine-treated (r = 0.47, p less than 0.08). When examined in a larger group (n = 46) of drug-free schizophrenics, this relationship was highly significant (r = 0.59, p less than 0.001). These clinical data are consistent with preclinical evidence indicating a functional interaction between CNS somatostatin and dopamine systems.