To determine the most effective route of administering prostaglandin E1 (PGE1) to inhibit warm ischemic liver damage, 90-min warm ischemia was established in a canine model. The dogs were divided into three groups according to the treatment given. Thus, group A (n = 10) was the control group which received no treatment, group B (n = 7) was administered PGE1 intravenously, and group C (n = 7) was administered PGE1 intraportally. PGE1 was continuously administered before and after the ischemia at a rate of 0.02 microgram/kg/min. The branched-chain amino acid to aromatic amino acid ratio in the hepatic vein, and the arterial ketone body ratio of acetoacetic acid to beta-hydroxybutyric acid, were examined to observe the metabolism of each amino acid and the oxidation-reduction ability of hepatocytes. Both ratios were maintained only in the group C dogs, three of which survived for over 3 days, whereas in groups A and B, all the dogs died within 24 h. The results of this study imply that the intraportal administration of PGE1 was more effective against warm ischemic liver damage than the intravenous administration of PGE1.