Further investigations of blood sugar control in the Wistar albino rat (Rattus norvegicus) and the golden hamster (Mesocricetus auratus), kept in the laboratory at 21°C have been made. Concentrations of plasma insulin in the fasting and the glucose loaded animal were determined, and it was shown that the hamster maintains a lower plasma insulin concentration than the rat, and secretes less insulin in response to glucose loading. It is concluded that the high fasting blood sugar level and poor glucose tolerance of the hamster result from a relative lack of insulin in this animal.