In the present study, nicotinamide adenine dinucleotide hydrogen phosphate-diaphorase (NADPH-d) histochemistry has been used as a marker for nitric oxide synthase. NADPH-d positive neuronal cell bodies and fibres have been localized in the pancreas of the mouse, rat, chick, kitten and monkey. In all these species, most of the NADPH-d labelled neuronal cell bodies in the ganglia were found in the interlobular and interacinar connective tissue while some were intimately associated with pancreatic ducts and blood vessels. There was, however, a gradation of innervation amongst the species. Only in the pancreatic islets of the mouse and monkey were NADPH-d positive neuronal cell bodies and nerve fibres detected in the formation of neuro-insular complexes. Besides labelling of neuronal cell bodies, nerve fibres and endothelial cells, the islet cells of the mouse, kitten and chick pancreas also gave a light positive reaction for NADPH-d activity. The nerve fibre and the outer core of the connective tissue of the Pacinian corpuscles in the kitten pancreas were also labelled for NADPH-d activity. It is concluded that nitric oxide may play an important role in the neural regulation of both exocrine and endocrine secretion and in controlling the activity of the blood vessels and ducts of the pancreas.