The influence of x-radiation and N-methyl-N'-nitro-N-nitrosoguanidine [(MNNG) CAS: 70-25-7] on intestinal metaplasia and gastric tumorigenesis was examined in 5-week-old male Crj:CD(SD) rats. The animals were treated either with two 10-Gy fractions of x-rays separated by 3 days for a total of 20 Gy to the gastric region and/or with MNNG orally for 4 months. Simultaneous treatment with x-rays and MNNG (group II) and MNNG only (group IV) induced gastric tumors in the majority of the animals. Sequential treatment with x-radiation and MNNG, either x-ray 2 months prior to MNNG (group I) or MNNG 2 months prior to x-ray (group III), resulted in a lower incidence of gastric tumors as compared with the incidence after treatment with MNNG alone. The frequencies of intestinal metaplasia in the x-irradiated groups (groups I and V) were significantly higher than those in group II, III, or IV. The incidence of intestinal metaplasia and of gastric tumor was inversely proportional. These results indicate that intestinal metaplasia does not play a role in the induction of gastric tumors by MNNG.