Although acetate, the main circulating volatile fatty acid in humans and animals, is metabolized at high rates by the renal tissue, little is known about the precise fate of its carbons and about the regulation of its renal metabolism. Therefore, we studied the metabolism of variously labeled [13C]acetate and [14C]acetate molecules and its regulation by alanine, which is also readily metabolized by the kidney, in isolated rabbit renal proximal tubules. With acetate as sole substrate, 72% of the C-1 and 49% of the C-2 of acetate were released as CO2; with acetate + alanine, the corresponding values were decreased to 49% and 25%. The only other important products formed from the acetate carbons were glutamine, and to a smaller extent, glutamate. By combining 13C NMR, radioactive and enzymatic measurements with a novel model of acetate metabolism, fluxes through the enzymes involved were calculated. Thanks to its anaplerotic effect, alanine caused a stimulation of acetate removal and a large increase in fluxes through pyruvate carboxylase, citrate synthase and the enzymes involved in glutamate and glutamine synthesis, but not in flux through a-ketoglutarate dehydrogenase. It is concluded that the anaplerotic substrate alanine not only accelerates the disposal of acetate but also prevents the wasting of the latter compound as CO2.