The physiological role of opioid peptides in the rat striatum was sought by intracellular recording in vitro. Excitatory synaptic potentials (mediated by glutamate or aspartate) and inhibitory synaptic potentials (mediated by GABA) were isolated pharmacologically and/or by positioning the stimulation electrode over the external capsule. Opioid agonists and antagonists selective for mu-, delta-, and kappa-receptors were applied by superfusion. Two main actions of opioids were observed. First, mu- and delta-selective opioids presynaptically inhibited the excitatory postsynaptic potential, whereas only delta-selective opioids decreased the inhibitory synaptic potential. Second, a small subpopulation of cells (not medium spiny neurons) were hyperpolarized by delta-selective agonists. The results indicate that the main action of opioids on striatal neurons is presynaptic inhibition of the corticostriate excitatory synaptic input.