Glucose homeostasis in a carnivorous (cat) and in an omnivorous (rat) animal fed a high-protein, low-carbohydrate (HP) diet followed a pattern similar to that previously found in carnivorous birds [Am. J. Physiol. 234 (Regulatory Integrative Comp. Physiol. 3): R115-R121, 1978]. Thus, compared to controls given carbohydrate-rich (HC) diets. HP-fed animals had, in the fed state, lower levels of blood sugar, lower concentrations of liver glycogen, and a tendency to reduced body glucose mass. During fasting, their liver glycogen was little mobilized and their glycemia remained remarkably constant. Accordingly, their body glucose mass remained unchanged. Gluconeogenic capacity, already high in the fed state, was not further activated by fasting. Phosphoenolpyruvate carboxykinase activity actually decreased in rat liver cytosol, remaining unchanged in cat liver cytosol and mitochondria. [3H]glucose turnover studies revealed that in HP-fed cats and rats glucose replacement rates in the fed state were as high as in HC-fed animals and that the resistance of their glycemia to fasting depended on the high gluconeogenic capacity and not on a more efficient reduction of glucose utilization.