After referring to experimental work previously published, which proved that a naturally occurring high fasting blood sugar is transmitted in animals as a Mendelian recessive character, cases are quoted which show that diabetes mellitus, or a tendency to it, in human beings may be transmitted in the same way, often through several generations of heterozygous normals. As a rule the patients in whom this type is met with are young, and the disease is progressive and severe. Predisposition to the milder variety met with in elderly people is apparently transmitted as a Mendelian dominant and is probably brought into evidence by acquired defects caused by faults of diet and general hygiene.