Specimens from the liver, kidney, stomach and various intestinal segments of BD IX male rats were analyzed for mitotic index (MI) and the proportion of karyorrhectic figures (karyorrhectic index, KI) at various intervals after a s.c. injection of 1,2-dimethylhydrazine. In the intestinal specimens, a severe drop in MI (maximum after 12--14 h) and rise in KI (maximum after 6 h) were observed, with all karyorrhectic figures found in the proliferative compartment of the mucosa. Within the large intestine, there was a good correlation between the KI of each segment and its susceptibility to further tumor development.