Nitric oxide synthase (NOS) is widely distributed throughout the nervous system and is found in neurons which produce nitric oxide (NO). In attempting to elucidate the biological roles of NO in neurotransmission, vasodilation, and in neurodegeneration, nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd) histochemistry has been widely used. NADPHd histochemistry and NOS immunoreactivity (NOS-IR) have been assumed to stain the same population of neurons. However, there have been numerous reports which suggest that this may not always be the case, and in all neuronal populations investigated, the coincidence of NOS and NADPHd must be unequivocally demonstrated. We have examined NADPHd histochemistry and NOS immunoreactivity in the human and rat retina and shown that these are 100% co-localised. Further, we have described the morphology of NADPHd and NOS-IR neurons in the human and rat retina and shown a close association of these neurons and their processes to the retinal vasculature. We have taken the NOS-IR to the ultrastructural level and have identified NOS-IR cells in close association with the basal lamina covering endothelial cells and pericytes of the retinal capillaries. We suggest that NO released from these neurons may be involved in the regulation of retinal microcirculation.