The initial stages of cellular and lymphatic invasion were studied in experimental carcinomas of rat-stomach induced by N-methyl-N'-nitro-N-nitrosoguanidine. In initial invasion, lack of the basement membrane and cytoplasmic projections into the adjacent connective tissues was observed. The mode of penetration of cytoplasmic protrusion into the stroma was also studied in cases of benign cells. As a result, invasion by carcinoma cells was thought to occur with its ameboid and proteolytic enzymic action against the surrounding tissues. The invasive mode into the lymphatic vessels was divided into the following 2 types; a destructive invasion type, and an endothelium type. It was interesting that differentiation and cellular adhensiveness of carcinoma cells was less in the course of invading into the lymphatic wall, but rose after that.