NADPH diaphorase activity is used as a histochemical marker for neuronal nitric oxide (NO) synthase; however, it remains unclear whether these activities are directly correlated in all tissues. In N1E-115 neuroblastoma cells, NADPH diaphorase activity was found primarily in the particulate fraction, whereas NO synthase activity was mostly soluble. Non-induced macrophages expressed significant NADPH diaphorase activity (which was mostly particulate) but virtually no NO synthase activity. Induction of macrophages produced marked increases in both NO synthase and NADPH diaphorase activities in the soluble and particulate fractions. In endothelial cells, both NO synthase and NADPH diaphorase activities were found mostly in the particulate fraction. Purified NO synthases from brain (type I), macrophages (type II), and endothelium (type III) all showed NADPH diaphorase activity; relative activities were: macrophage > endothelium > brain. These data indicate that all known NO synthases are NADPH diaphorases; however, NO synthases represent only a fraction of total cellular NADPH diaphorase activity and these activities are not always co-localized.