Liver regeneration after omeprazole (OMP) or famotidine (FAM) administration was examined in 66% hepatectomized rats. The regeneration was evaluated by the liver weight as a percentage of body weight (LRR) and the proportion of hepatocytes in mitosis per 1,000 counts (MI). Administration of OMP 0.4 mg/kg per day for 3 or 7 days suppressed LRR and MI 3 and 7 days after hepatectomy. However, the administration of FAM 0.8 mg/kg per day for 3 or 7 days did not change either LRR or MI. Increased gastrin levels in the blood were seen only after OMP administration. The food intake was unchanged by OMP or FAM, but FAM increased water intake. The liver functional score, glutamic pyruvic transaminase and alkaline phosphatase in the blood all increased with OMP, but FAM had no apparent effect on the hepatic or renal function. These observations suggest that a large dosage of OMP suppresses liver regeneration, while FAM appears to have no meaningful effect on regeneration.