Administration of dimethylhydrazine (DMH) to adult rats by two subcutaneous injections each of 120 mg(base)/kg body weight and spaced 10 days apart resulted in the development of renal tumours in over 90 per cent. of treated animals at 30 weeks after injection. The tumours were frequently bilateral and multiple, and had the structure of the "mesenchymal" tumours which occur in a high incidence following treatment with nitrosamines. The incidence of tumours both in the small intestine and in the colon was lower than that experienced using a weekly DMH treatment schedule.