Data are given to support the hypothesis that the symptom complex of diabetes may be produced in two ways, either by deficiency of insulin, the insulin sensitive type, or by lack of an unknown factor which sensitises the tissues to insulin, the insulin insensitive type. Simultaneous administration of insulin and glucose produced little rise in blood sugar in insulin sensitive patients, while marked hyperglycaemia resulted in insulin insensitive cases. In the latter group no marked fall in venous blood sugar was effected by insulin. Insulin sensitive patients seemed to tolerate high carbohydrate diets well.-A. Lyall.