The effects of prolonged administration of aminoguanidine on gastric carcinogenesis induced by N-methyl-N-nitro-N-nitrosoguanidine (MNNG), the ornithine decarboxylase activity of the gastric wall, and the labeling index of the gastric mucosa were investigated in inbred Wistar rats. The rats received 12.5 or 25.0 mg/kg body weight of aminoguanidine s.c. every other day after oral MNNG treatment for 25 weeks. Prolonged administration of aminoguanidine at 25.0 mg/kg body weight, but not at 12.5 mg/kg body weight, significantly increased the incidence of gastric cancers in experimental week 52. However, it did not influence the histologic types of gastric cancers. Aminoguanidine at 25.0 mg/kg body weight also significantly increased the ornithine decarboxylase activity of the antral portion of the gastric wall and the labeling index of the antral epithelial cells. These findings indicate that aminoguanidine enhances gastric carcinogenesis and suggest that its effect may be related to increased proliferation of antral epithelial cells.