We evaluated ceruletide, a cholecystokininlike peptide, in a double-blind, placebo-controlled study of 20 male chronic schizophrenic patients. After baseline investigations, 10 patients received 0.3 microgram/kg body weight ceruletide, and 10 patients received placebo (normal saline) intramuscularly once weekly for 3 consecutive weeks. Psychopathology was rated on the Brief Psychiatric Rating Scale and the Nurses' Observation Scale for Inpatient Evaluation. Blood was drawn on the same days for estimation of norepinephrine, epinephrine, beta-endorphin, cortisol, and prolactin. There were no significant changes in biochemical parameters. With regard to psychopathology, no significant differences in behavioral ratings were found between the ceruletide- and placebo-treated groups. Furthermore, there was no changes in either positive or negative symptoms of schizophrenia secondary to ceruletide. Contrary to uncontrolled studies, we failed to show antipsychotic properties of ceruletide.