The enzyme NADPH-diaphorase (NADPH-d; a marker of NO producing or nitrergic neurons) and the neuropeptide VIP have been detected in the nerve structures of the hen oviduct by histochemical and immunohistochemical techniques performed on cryostatic sections and whole mount preparations. In the upper four segments of the oviduct, i.e. the infundibulum, magnum, isthmus and shell gland, nitrergic and VIP-positive nerve structures were particularly numerous in the intermuscular and mucosal layers, and were represented by fibres and cell bodies showing mainly a perivascular distribution. Functionally, such perivascular structures were related to the blood flow regulation. Different types of nitrergic pyrenophora were recognized in the walls of the shell gland on the basis of their peculiar morphology. In the distal zone of the oviduct, the vagina, nitrergic and VIP-positive nerve fibres were widely diffused in the circular muscle, which was particularly thick in this segment. The source of at least part of such fibres was probably represented by large nerve cell bodies scattered in the layer and containing NO and VIP colocalized. Functionally these cells retained inhibitory motor neurons causing relaxation of the vaginal smooth muscle.