The fine structure of the hepatic artery of dogs was studied at electron microscopic level. The majority of nerve fibres innervating the artery was found at the adventitial-medial border of the vessel, but some of them entered the middle part of the media. The average distance of nerve to muscle fibre was found to measure 450 nm. Three types of nerve profile could be observed, viz. adrenergic, cholinergic and cholinergic-peptidergic as suggested by the morphology of their vesicles. It is suggested that the cholinergic and cholinergic-peptidergic nerve endings probably modulate the release and the constrictor effect of the main neurotransmitter noradrenaline in the hepatic artery.