To delineate the cellular sites for the motor effects of opiates acting at the mu opioid receptor (MOR) in the rat caudate-putamen nucleus, we examined the ultrastructural immunogold and immunoperoxidase labeling of an antipeptide antiserum specific for the MOR. We also combined these labeling methods to examine the subcellular relationship between the MOR and the endogenous opioid peptide, Leu5-enkephalin (LE). By light microscopy, MOR-labeling was seen in a heterogeneous patchy distribution. Electron microscopic analysis of these patches showed that more than 80% of the total neuronal profiles (n = 1,586) containing MOR-like immunoreactivity (MOR-IR) were dendrites and dendritic spines. The remaining labeled profiles included a few perikarya and many axon terminals. MOR-IR was predominantly localized to extrasynaptic plasma membranes of dendrites, and to both synaptic vesicles and plasma membranes in terminals. Ten percent of the total MOR-labeled terminals (n = 272) formed asymmetric synapses with unlabeled or MOR-labeled dendritic spines. Terminals containing LE-IR formed synapses, in almost equal proportions, on MOR-labeled dendrites and dendritic spines, while over 80% of the unlabeled terminals formed synapses on MOR-labeled dendritic spines. Moreover, colocalization of MOR- and LE-IR was often seen in both dendrites and terminals. These results indicate that in patch compartments of the caudate-putamen nucleus, the MOR is mainly involved in extrasynaptic modulation of spiny neurons, including those that contain LE. In addition, the findings provide a cellular basis for presynaptic opioid modulation of neurotransmitter release through MOR located on axon terminals.