Diabetes (db), which occurred in an inbred strain of mouse, is inherited as a unit autosomal recessive and is characterized by a metabolic disturbance resembling diabetes mellitus in man. Abnormal deposition of fat at 3 to 4 weeks of age is followed shortly by hyperglycemia, polyuria, and glycosuria. Accompanying morphological changes in the islets of Langerhans suggest neogenesis to compensate for insulin depletion.