1,2-dimethylhydrazine (DMH) is widely used to induce colorectal tumours in rodents. Some of the animals develop ear as well as colorectal tumours. Rats with large, ulcerated ear tumours are usually sacrificed before the completion of the experiment. In this experiment, fourty-six male Spraque-Dawley rats were injected with 1,2-dimethylhydrazine (21 mg/kg body weight) once a week for 27 weeks to study the histogenesis of colorectal carcinoma. Thirty-six developed ear tumours. Fourteen of the 36 tumours were larger than 2 cm in diameter. These developed between 20-26 weeks and were surgically excised 1-5 weeks later. Four rats died postoperatively. The surgical removal of large ear tumours permitted the completion of the large bowel experiment on schedule (i.e. 27 weeks) in 10 (28%) of the 36 rats with ear tumours.