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A histopathologic studp on the experimentally induced mesenchymal neoplasms of the stomach and small intestine by peroral administration of N-Methyl-N´-Nitro-N- Nitrosoguanidine(MNNG) was carried out to clarify the morphologic characteristics .and nature of their tumorigenesis. A total of 220 male rats were divided into two groups; control group of 83 Wistar .rats and experimental group(MNNQ group) of 86 Wistar and 51 Fisher rats that were .administered with MNNG at the level of 100 ng/ml in drinkiag distilled water ad lib- itum for a period of 37 weeks. The control group was fed with distilled water. After 87 weeks of exposure experimental animals were sacrificed and autopsied. Followfng the macroseopic examination of entire ga´strointestinal traet, stamach and amall intestine were serially cut, embedded in paraffin and were examined histopatho- logicallv. The results were as follows; 1. In MNNG administered group the mesenchymal lesions including true neoplastic growth developed in 52 cases(37. 9%), of which 37 were found in the stomach and 15 in the small bowel. Among these mesenchymal lesions submucosal fibroplastic lesions was the most common one being 75.2%. And the remainders consisted of truelp neo- plastic of both benign and malignant neoplasms, i.e., 3 leiomyomas, I hemangioma, 1 lymphangioma, I fibroma, 6 angiosarcomas, 4 leiom#yosarcotnas, 1 ntalignant achwann- ama and I sarcoma of unclassified type. 2. Seventeen cases(50.0%) of the submucosal fibroplastlc lesions was found as isolated lesion without aacompanying pathological process, and was confined witbin the submu- cosal tissue of the gastrointestinal trset. And the rerasinder was associated with
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