Plasma concentrations of ammonia were examined following prostaglandin E1 (PGE1) administration in 16 patients who received hepatectomy for hepatocellular carcinoma. PGE1 at 0.01-0.04 microgram/kg/min was administered for approximately 24 hr during the perioperative periods. The ammonia concentration was reduced a day after the operation following intravenous administration of PGE1 in cirrhotic patients compared to noncirrhotic patients with or without PGE1 injection. Serum liver function scores, glutamic pyruvic transaminase, and total bilirubin concentrations were decreased after PGE1 administration. A similar ammonia response due to PGE1 was obtained after operation in cirrhotic patient when PGE1 0.5 microgram/kg was administered intravenously for 30 min. These results suggest that PGE1 administration is useful in reducing the enhanced ammonia concentration associated with hepatic resection, and it is particularly effective in the case of liver cirrhosis.