Neurons in the monkey cerebral cortex containing nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) can he divided into two distinct types, both nonpyramidal. Type I neurons have a large soma (diameter 20–50 μm), a dense NADPH-d histochemical reaction. and are distributed throughout the cortex, but mainly in the subcortical white matter, and are mostly aspiny. Type II cells have a small soma (< 20 μm) with light NADPH-d reactivity and are distributed primarily in the supragranular layers, particularly layers II and upper III. The numerical density of type II cells is much greater than that of type I. Type I neurons also stain for GABA and a few intracortical type I cells contain calbindin. All type II cells found here are colocalized with both GABA and calbindin. Neither type I nor type II cells are stained for parvalbumin.

Together with previous observations that almost all cortical NADPH-d cells in various subprimates are like type I cells, we suggest that type II cells may form a group of NADPH-d-rich neurons differentiated in higher mammalian cortex from a subpopulation of calbindin-containing GABAergic interneurons, and these nitric oxide-synthesizing cells may play a role in control of intracortical neuronal activity characteristic of higher cerebral functions in advanced mammals.