The incidence and course of development of diabetes was studied for 12 months after subtotal pancreatectomy in five strains of rats. Operations were performed on 5-7-week old-male animals weighing 100-140 g, and, subsequently, rats with fasting blood glucose levels greater than 8.3 mmol/l were considered to be diabetic. In FΓΌllinsdorf Albino rats, the proportion of diabetic animals gradually increased during the course of the experiment to a maximum of 83%. In Holtzman and Piebald rats, the highest incidence (58% and 28% respectively) was observed 2-3 months after surgery, after which partial remission of their diabetes occurred. Spontaneously hypertensive rats completely recovered from an initially mild diabetic state within 6 months of the operation. In Lewis rats, only a small number of animals (15%) became diabetic towards the end of the experiment. The severity of diabetes also differed considerably between strains. The pancreatic insulin content in diabetic rats was 2%-7% and in operated non-diabetic rats 17%-26% of values in non-operated control animals of the same age and strain. However, in the operated non-diabetic rats, glucose tolerance and the insulin response to glucose were also impaired. It is concluded that there are marked inter-strain differences in rats concerning the incidence and development of diabetes following subtotal pancreatectomy.