The peptide (D-Pro4)-beta-casomorphin1-5 is a potent and long acting analgesic. Furthermore it is able to antagonize apomorphine-induced behavioral patterns, which are preferentially used as screening methods to detect dopaminolytic or neuroleptic properties. Because all of these tests do not exclude interaction of drugs with transmission systems other than the dopaminergic, biochemical studies were undertaken to estimate possible influences of the opioid peptide on processes of dopaminergic, serotonergic, and cholinergic transmission systems. In lower concentrations (D-Pro4)-beta-casomorphin1-5 enhances the potassium-stimulated release of acetylcholine from hippocampal slices and the basal overflow of dopamine from striatal slices. In high concentrations an augmentation of the potassium evoked release of dopamine and a reduction of the binding of [3H]spiperone on dopaminergic and serotonergic striatal receptors could be observed. These biochemical findings are discussed with regard to the behavioral patterns induced by this opioid peptide.