Intraperitoneal injection of FK 33-824 produced apomorphine-like stereotyped behavior in rats. Antagonism of this stereotypy by naloxone and neuroleptics suggests that FK 33-824 can activate opiate and dopamine receptors in the brain. Because increased dopaminergic neuronal activity is thought to be involved in schizophrenia and dopamine-mediated stereotypy has been used as an animal model for this illness, these results are consistent with an involvement of endogenous opiate-like peptides in schizophrenia. This involvement provides a possible mechanism for the reported improvement in schizophrenic psychosis produced by naloxone.