The distribution of the enzyme dihydronicotinamide adenine dinucleotide phosphate (NADPH) diaphorase was examined in the striate cortex of the rhesus monkey. The pattern of activity in the neuropil matched that of cytochrome oxidase in adjacent sections and the enzymes were similarly modulated by monocular deprivation. Scattered individual cells were also intensely positive for NADPH diaphorase. Labelled cells were most common in the white matter and layers 2 and 3; they were least common in layers 4 and 5. Diaphorase cells were morphologically diverse, but no pyramidal or spiny cells were labelled. Labelled cells often had multiple varicose processes, which extended laterally for over 1 mm. Although the function of this enzyme is unknown, the morphology and distribution of the diaphorase-positive cells resembles published reports of cortical cells containing somatostatin, avian pancreatic polypeptide, and neuropeptide Y-like immunoreactivity, and NADPH diaphorase is colocalized with these substances in the rodent striatum (S.R. Vincent, O. Johansson, T. Hökfelt, L. Skirboll, R.P. Elde, L. Terenius, J. Kimmel, and M. Goldstein, J. Comp. Neurol. 217:252-263, '83).