Beta-endorphin, cholecystokinin and vasoactive intestinal peptide were measured in peripheral blood mononuclear cells of healthy controls, and schizophrenic patients at the first diagnosis before any treatment and after 2 or 15 d of treatment with haloperidol. Beta-endorphin concentrations were similar in controls and untreated patients, and increased with treatment. Cholecystokinin concentrations were higher in patients than in controls, and decreased during treatment. Vasoactive intestinal peptide was lower in patients and did not change with treatment. These observations are consistent with measurements of the same peptides in autopsy samples or cerebrospinal fluid. Peripheral blood mononuclear cells might be an useful tool for the study of some neuropeptides in brain.