The metabolism of variously labeled [13C]- and [14C]glucoses, used at a physiological concentration (5 mM), has been studied in isolated rabbit kidney tubules both in the absence and the presence of NH4Cl. When present as sole exogenous substrate, glucose was metabolized at high rates and converted not only into CO2 and lactate but also, in contrast to a previous conclusion of Krebs (Krebs, H.A. (1935) Biochem. J. 29, 1951-1969), into glutamine. Absolute fluxes through enzymes of glycolysis and gluconeogenesis and of enzymes of three different cycles operating simultaneously were assessed by using a novel model describing reactions of glucose metabolism in conjunction with the 13C NMR and, to a lesser extent, the radioactive data obtained. The presence of NH4Cl (5 mM) caused a large stimulation of glucose removal and a large increase in lactate, glutamine, and glycerol 3-phosphate accumulation. Under this condition, the stimulation of glutamine synthesis was accompanied not by an activation of citrate synthesis but by an inhibition of flux through alpha-ketoglutarate dehydrogenase. The resulting depletion of citric acid cycle intermediates was compensated by anaplerosis at the level of pyruvate carboxylase. The "futile" cycle involving oxaloacetate, phosphoenolpyruvate, and pyruvate, which was intense in the presence of glucose alone, was greatly stimulated by the addition of NH4Cl.