The parenteral administration of 1,2-dimethylhydrazine to rats caused the development of colonic neoplasms in about 90% of animals by 24--30 weeks of treatment. Usually there were multiple tumours with a mean of 2.7 per rat. The lesions have been classified histologically into adenomata (26% of all tumours) and carcinomata, the latter showing varying degrees of differentiation. No completely anaplastic tumours were seen, and there were none originating in connective tissue. The distributions of the different tumour types along the length of the colon varied. The more benign lesions were situated predominantly in the distal half of the colon, while the poorly differentiated adenocarcinomata were concentrated in the proximal third of the colon. There was good evidence to suggest that adenomata often progressed to frank malignancy in the distal colon. In the proximal part, however, it appeared that tumours frequently developed de novo as poorly differentiated carcinomata. Perhaps regional variations in the kinetic organisation of the normal colonic mucosa somehow influence the nature of the neoplastic change induced by DMH, thus accounting for the differences in tumor distribution. After 24 weeks of DMH treatment there was only a small increase in the mean number of tumours per rat.