A simple enzyme histochemical technique is described that detects various distinct populations of neurons in the brain. These neurons contain an extremely high activity of an endogenous enzyme, NADPH-diaphorase, that can reduce the dye nitro blue tetrazolium to a bright blue reaction product. Some of the major groups of neurons detected by this technique occur scattered throughout the neocortex, in the striatum and in the laterodorsal tegmental nucleus. The soma, dendritic trees and fiber networks of the positive neurons are stained in their entirety. Thus this simple, reliable technique can be used to obtain a Golgi-like image of particular groups of neurons in various regions of the brain.