The effectiveness of NMDA antagonists in modulating the motor and motivational effects of opiates is attributed, in part, to functional associations involving NMDA receptors and micro-opioid receptors (MORs) in the shell of the nucleus accumbens (Acb). To determine the subcellular sites for potential functional interactions between opiate ligands and NMDA receptors in this region, we examined the ultrastructural localization of antipeptide antisera against MOR and the R1 subunit of the NMDA receptor in the Acb shell of the adult rat brain. MOR-like immunoreactivity (MOR-LI) was seen primarily in dendrites, whereas NMDAR1-like immunoreactivity (NMDAR1-LI) was detected more often in axon terminals forming asymmetric synapses. In these profiles, MOR labeling was localized mainly to extrasynaptic plasma membranes, whereas NMDAR1-LI was associated with both synaptic and extrasynaptic sites. Of 307 MOR-labeled processes, 17.9% of the dendrites and 9.4% of the axon terminals also contained NMDAR1-LI. In addition, 24.7% of the dendrites containing only MOR-LI were apposed to NMDAR1-labeled axons or terminals. We conclude that in the shell of the Acb, the output of single neurons can be dually modulated by (1) activation of MOR and NMDA receptors in the same dendrites or (2) combined activation of presynaptic NMDA receptors in afferents contacting dendrites containing MOR. In addition, the colocalization of MOR and NMDAR1 in certain axon terminals in the Acb suggests their dual involvement in the presynaptic release of neurotransmitters in this region.