Controversy persists concerning the distribution of intrahepatic arterial termination. Apart from nourishing structures in the portal tract, several authors have suggested arterial supply to some isolated vascular beds that bypass the parenchyma, but this was not verified morphologically. In the present study, the existence of an isolated hepatic artery unaccompanied by a portal vein or a bile duct was shown in livers of pigs and other mammals including the dog, seal, ox, horse, and man. After colored media injection, liver blocks were sampled near the hepatic capsule and hepatic vein and subject to histologic assessment. The hepatic artery dissociated from superficial portal tract, ramified, and anastomosed extensively, forming a capsular plexus that drained at places into peripheral sinusoids in which resistive structural elaborations were observed. The artery also dissociated and fed an intramural plexus (vasa vasorum) of hepatic veins. This was collected into a vessel that penetrated the intima into the vein's lumen. In smaller sublobular veins, the plexus emptied into adjacent sinusoids; in central veins, the plexus was poorly developed. In canine and phocine livers, which have a special throttling musculature in sublobular veins, the isolated arterioles were numerous; in bovine and equine livers, which have a thick fibrous investment, the capsular plexus was well-developed. Occurrence of the isolated artery also was confirmed in the human but not in the rodent liver. In conclusion, the isolated artery is responsible for isolated vascular beds, in which the transintimal pathway can bypass the lobular sinusoids.