Abstract
A total of 30 inbred Wistar rats were orally administered 70 microgram/ml solution of N-methyl-N'-nitro-N-nitrosoguanidine (MNNG) for 35 weeks and then tap water for the following 20 to 30 weeks. Four of the 20 females and two of ten males developed carcinoids in the glandular stomach, but no metastasis could be found. Carcinoids developed most frequently in the fundic portion along the greater curvature. Histologically, these tumors were medullary anaplastic carcinomas containing two different endocrine cell populations. The first cell type was argentaffin having the electron-dense, somewhat pleomorphic secretory granules (437-810 nm) and the second type was argyrophil having round granules with a dense core and a pale halo (550 nm). None of these tumors showed endocrine immunoactivity for gastrin, somatostatin, insulin, glucagon, and enkephalin. One of these gastric tumors developed into scirrhous carcinoma, but differentiated adenocarcinoma could not be seen in the glandular stomach.