Nerve fibers supplying the Duvernoy's gland, a venom-secreting oral gland, of the Japanese colubrid snake, Rhabdophis tigrinus, were examined by formaldehyde-induced fluorescence (FIF) and acetylcholinesterase (AChE) histochemistry, immunohistochemistry, and electron microscopy. The innervation by the FIF fibers was rather meager and was restricted to the area around the arteries localized in the interlobular connective tissue. The AChE-reactive fibers, in contrast, were abundantly supplied all over the gland, especially to the lobule consisting of secretory units. Peptide histidine isoleucine (PHI)(1-15)-like immunoreactive fibers were also detected in the gland and proved identical to the AChE-reactive fibers. The reactivities for both AChE and PHI (1-15) were particularly prominent around the blood capillaries distributed in the lobules. Under the electron microscope, nerve fibers were frequently seen to terminate near capillaries subjacent to the secretory unit. Those nerve terminals containing small clear vesicles and dense core granules were devoid of Schwann cell coverage on the side facing the blood capillary. These features suggest that the nerve terminals may possibly release at least a portion of their secretory contents into the blood as neurohormones.