• A double-blind study of the behavioral effects of short-term naloxone hydrochloride administration was performed in 32 schizophrenic and 26 manic patients in a World Health Organization collaborative project. There was a significant naloxoneassociated reduction in overall physician-rated symptoms in schizophrenic patients concurrently treated with neuroleptic medication (N = 19) but not in medication-free schizophrenics (N = 13). Physician ratings of auditory hallucinations showed significant naloxone-associated improvement for the total schizophrenic population, while self-ratings of auditory hallucinations showed improvement only in neuroleptic-treated schizophrenics. While further studies are needed to delineate these effects as to clinical significance, they may bear etiological implications for the psychobiology of schizophrenia, including the possibility of synergistic effects of dopamine and endorphin blockade. Naloxone produced no significant behavioral effects in manic patients. These findings are discussed with relationship to the hypotheses of endorphin involvement in schizophrenia and mania.