In our colony, yellow obese mice were longer than nonobese littermates and yellow males were heavier than yellow females. Weight gain was greatest on high fat diets. If the yellow mice were given a possibility to exercise (activity cages), they lost weight. Resistance to cold was good. Fasted blood glucose levels were normal, fed levels frequently elevated in males. Yellow male and female mice showed a degree of insulin resistance; yellow males showed a marked hyperglycemic response to growth hormone, ACTH, cortisone, and glucagon. Blood total lipids were elevated in yellow mice, with the females exhibiting hypercholesterolemia. As in other forms of ‘metabolic’ obesity, blood ketones were decreased by an 18 hours fast.