The effect of hydrochlorhydria caused by vagotomy on carcinogenesis in the glandular stomach of male rats was studied. Group A (35 rats): After N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) 80 mg/l solution was orally administered during the first 15 weeks of life, vagotomy was performed. Group B (35 rats): After the oral administration of MNNG 80 mg/l solution during the first 15 weeks of life, laparotomy was done. Group C (10 rats): As the control, vagotomy was undertaken at the 15th week of life. Group D (10 rats): As the control, laparotomy was done at the same time. At the 52nd week, all surviving rats were autopsied, and gastrin cell counts and body weight were ascertained. The incidence of adenocarcinoma was 62% in Group A, 32% in Group B (p less than 0.05) and nil in Group C and D. These results strongly support the view that the hypochlorhydria plays the role of a promoting factor in producing gastric carcinoma.