In a morphogenetic study of experimental tumors of the intestine, 556 male noninbred albino rats were given weekly sc injections of 21 mg 1, 2-dimethylhydrazine dihydrochloride/kg body weight and were killed at different intervals (10-70 rats/wk) after the beginning of treatment. Intestinal carcinomas developed in almost all rats surviving 5 months after the beginning of the experiment. In addition, intestinal tumors induced in 800 rats of both sexes were examined. Tumor development began with a widening of the proliferative zone within the crypts, an indication of enterocyte differentiation disorders. The ensuing in situ carcinoma became superficial cancer capable of invading the lamina propria of the mucosa. Its continued growth caused the tumor to penetrate the tunica muscularis mucosae into the underlying layers of the intestinal wall. Thus experimental intestinal adenocarcinomas developed de novo or, at least, were not preceded by adenomatous polyps. Signet-ring cell carcinomas began with the accumulation of goblet cells in cells of signet-ring appearance; later they ruptured the basal membrane and infiltrated the surrounding tissues. The proposed scheme of morphogenesis of experimental tumors was correlated with current concepts of rectal tumor development in man.