This paper provides a comprehensive morphological description of local circuit neurons in the medial prefrontal cortex (mPFC: areas 24a, 24b, 24c, 25 and 32) of the monkey. Cortical interneurons were identified immunocytochemically by the expression of the calcium binding proteins calretinin (CR), parvalbumin (PV) and calbindin D-28k (CB). Interneurons were also identified using GABA immunocytochemistry. The areal and laminar distributions of CR, PV, and CB cells were consistent across mPFC; their morphological characteristics identified them as local circuit neurons. Throughout layers 2-6: CR immunoreactivity labelled double bouquet and bipolar neurons, PV was localised in large and small basket neurons and in chandelier (axoaxonic) cells, while CB immunoreactivity was present in double bouquet, Martinotti, and neurogliaform neurons. In addition, some cells in layer 1 (including Cajal-Retzius neurons) were CR immunoreactive. Calbindin immunoreactivity also labelled a population of large nonpyramidal neurons deep in the cortex. Other types of CR, PV and CB cells were also immunolabelled. A small population of layer 3 pyramidal cells was weakly CB immunoreactive. Peak cell densities occurred in layer 2/upper layer 3 for CR+ neurons and in upper to midlayer 3 for CB+ cells. PV+ neuron density peaked in midcortex. These observations support and extend a similar study of monkey prefrontal cortex (Condé et al.  J. Comp. Neurol. 341:95-116). The morphologies and combined cortical depth distributions of CR+, PV+, and CB+ neurons were similar to GABA-immunolabelled cells. Local circuit neurons in mPFC displaying NADPH diaphorase activity composed less than 0.25% of the total neuron population, and were distributed in two horizontal strata, in mid- to lower layer 3 and in lower layer 5/upper layer 6. CR, PV and CB immunoreactivity was colocalised in NADPH diaphorase-reactive neurons. The interrelationships between CR+, PV+ and CB+ neurons were investigated using dual immunocytochemistry. CR+ puncta were found to be closely associated with the cell bodies and proximal processes of PV+ neurons, whereas CR+ puncta were located more distally over processes from CB+ cells. Additionally, PV+ puncta were found closely apposed to PV+ somata and processes and CR+ puncta abutted against CR+ cell bodies. The companion paper (Gabbott and Bacon  J. Comp. Neurol.) presents quantitative data regarding the areal and laminar distributions of the identified cell classes in mPFC. Such data provide a realistic structural framework with which to investigate neuronal operations in monkey mPFC.