Slices from kidneys of normal, alloxan diabetic, and cortisone treated rats have been incubated with pyruvate-2-C14 and the incorporation of C14 into various products has been determined. Pyruvate uptake, and the incorporation of C14 into glucose and lactate was greater than normal in the diabetic and cortisone treated rat kidney slices. Incorporation of C14 into glycogen was increased above normal in the cortisone treated slices. There was a several fold increase in glucose content above normal in the incubation with diabetic and cortisone treated kidney slices. A large percentage of this newly formed glucose was synthesized from pyruvate carbon as measured by the isotopic techniques. From these in vitro experiments, to the extent of their in vivo physiological relevance, it is concluded that in diabetes mellitus and as a consequence of cortisone administration the metabolism of pyruvate in the kidney is significantly altered from the normal and gluconeogenesis is increased.