The genetically diabetic mouse ( db/db ) exhibits hyperphagia, progressive weight gain, hyperglycemia and hyperinsulinemia during the first four months of life. In an attempt to define certain features of the metabolic efficiency portrayed in this mutant species, the following studies were undertaken. Beginning at one month and continuing for twelve weeks, diabetics were subjected to controlled food intake (eating permitted 1–3 hours per day) to prevent obesity and maintain their weight identical to non-diabetic heterozygotes (+/ db ). Control diabetics (adlib food intake) consumed approximately 40 grams per week. Non-diabetics ate 20 grams per week whereas diet restricted diabetics consumed 10 grams/week. Their ability to maintain a weight identical to non-diabetics while consuming half the food intake was evidence of their metabolic efficiency. The diet restricted diabetics had normal blood glucose and water intake, but remained hyperinsulimic. In a second experiment, a similar protocol was utilized, except that diet restricted diabetics were given progressively increasing doses of oral diazoxide to suppress insulin. Achieved plasma insulin levels were comparable in diet restricted diabetics and heterozygotes, but food consumption in the treated group was unchanged, indicating persistence of their metabolic efficiency. In a third experiment, diabetics fed ad lib received high dose oral diazoxide and were compared with heterozygote non-diabetics. The food intake and growth pattern of the diabetics was not altered, as compared to control diabetics in Experiment 1, Plasma insulin was not suppressed, but water intake approached normal levels suggesting improved glucose control. It is concluded that insulin is not the only factor contributing to the metabolic efficiency in the db/db mouse. Hyperinsulinemia is a primary and fundamental abnormality in this species. High dose oral diazoxide appears to improve glucose tolerance in the fed state, and may act by increasing the number or sensitivity of insulin receptors.