The distribution of the enzyme nicotinamide adenine dinucleotide phosphate (NADPH) diaphorase, which is identical to nitric oxide synthase (NOS), was examined in cortical area 17 and the 17/18a border region in the brain of the golden hamster. The activity of the enzyme was present as a network of processes and in special populations of neurons in the visual cortex. The dense enzyme-positive network exhibited numerous varicosities distributed throughout the cortex. The prominent orientation of the processes in layer I and the white matter are parallel to the surface of the brain, but those in layers II-IV are perpendicular to the surface of the brain. However, the processes in layers V and VI seem to run randomly. The NADPH diaphorase-positive cells could be divided into two classes: heavily stained neurons and lightly stained neurons. For the lightly stained NADPH diaphorase-positive neurons, only the cell bodies could be observed, whereas for the heavily stained neurons, the cell bodies and their varicosity-carrying dendrites and, occasionally, the smooth, thin and weakly stained axons were visible. The heavily stained neurons were morphologically diverse, but no pyramidal or spiny neurons were found. Multipolar and bipolar neurons were located throughout the depth of the cortex, including the white matter, more frequently in layers V and VI. Occasionally, monopolar neurons were found in layer VI. Callosal projecting neurons in the visual cortex were labeled retrogradely with the use of FluoSpheres applied at the opposite visual cortex, but these neurons did not co-localize with the NADPH diaphorase-positive neurons, suggesting that the callosal projecting neurons and NADPH diaphorase-positive neurons belong to two populations of cells in the visual cortex.