Prefrontal corticostriatal afferents to the caudate-putamen nucleus (CPN) have been implicated in motor and cognitive functions that are subject to opioid modulation through the mu-opioid receptor (MOR). We examined the cellular basis for this modulation by combining anterograde transport of biotinylated dextran amine (BDA) following injections into the rat prefrontal cortex with immunocytochemical detection of MOR in patch compartments of the CPN. The BDA-labeled neurons in deep layer V and layer VI of the dorsal part of the anterior cingulate cortex and the medial agranular cortex projected bilaterally, with an ipsilateral predominance, to MOR-enriched patches in the dorsomedial and dorsocentral CPN, respectively. BDA-labeled terminals often apposed MOR-immunoreactive dendrites and perikarya but formed exclusively asymmetric, excitatory-type synapses mainly with dendritic spines. Of the total anterogradely labeled axon terminals forming asymmetric synapses, 40% (151 of 377) were with MOR-labeled spines, and 58% (220 of 377) were with unlabeled dendritic spines. In addition, immunogold-silver particles for MOR were seen in 14% (134 of 938) of all BDA-labeled axons and axon terminals. These dually labeled axon terminals also formed asymmetric synapses with dendritic spines that contained MOR immunoreactivity. The proportions of BDA-labeled axon terminals forming asymmetric synapses with MOR-labeled or unlabeled spines were similar in the CPN ipsilateral and contralateral to the cortical injections. These results suggest that, in patch compartments of the CPN, MOR plays a critical role in postsynaptic response to, but also in presynaptic modulation of, prefrontal corticostriatal excitation.