Summary Adult male rats, previously force-fed a high fat diet by stomach tube in amounts sufficient to maintain body weight, were less sensitive to the action of injected insulin than similarly-treated animals previously force-fed equi-caloric amounts of a high carbohydrate diet. The decreased sensitivity manifested itself primarily as a markedly increased rate of recovery from insulin hypoglycemia. The most likely direct cause of the increased insulin tolerance in the fat-fed animals is the higher level of liver glycogen after a 30-36 hour fasting period.