1. The pathways and the fate of glutamate carbon and nitrogen were investigated in isolated guinea-pig kidney-cortex tubules. 2. At low glutamate concentration (1 mM), the glutamate carbon skeleton was either completely oxidized or converted into glutamine. At high glutamate concentration (5 mM), glucose, lactate and alanine were additional products of glutamate metabolism. 3. At neither concentration of glutamate was there accumulation of ammonia. 4. Nitrogen-balance calculations and the release of 14CO2 from L-[ 1-_4C]glutamate (which gives an estimation of the flux of glutamate carbon skeleton through a-oxoglutarate dehydrogenase) clearly indicated that, despite the absence of ammonia accumulation, glutamate metabolism was initiated by the action of glutamate dehydrogenase and not by transamination reactions as suggested by Klahr, Schoolwerth & Bourgoignie [(1972) Am. J. Physiol. 222,[813][814][815][816][817][818][819][820] and Preuss [(1972) Am. J. Physiol. 222, 1395-13971. Additional evidence for this was obtained by the use of (i) amino-oxyacetate, an inhibitor of transaminases, which did not decrease glutamate removal, or (ii) L-methionine DL-sulphoximine, an inhibitor of glutamine synthetase, which caused an accumulation of ammonia from glutamate. 5. Addition of NH4Cl plus glutamate caused an increase in both glutamate removal and glutamine synthesis, demonstrating that the supply of ammonia via glutamate dehydrogenase is the rate-limiting step in glutamine formation from glutamate. NH4Cl also inhibited the flux of glutamate through glutamate dehydrogenase and the formation of glucose, alanine and lactate. 6. The activities of enzymes possibly involved in the glutamate conversion into pyruvate were measured in guinea-pig renal cortex. 7. Renal arteriovenousdifference measurements revealed that in vivo the guinea-pig kidney adds glutamine and alanine to the circulating blood.Since the initial work of Krebs (1935), it is known that, in contrast with kidney-cortex slices of dog, cat, pig, rat, sheep and pigeon, slices of guinea pig and rabbit do not release ammonia when incubated with glutamate. Krebs concluded that this was so in both the latter species because the ammonia formed by the deamination of glutamate via glutamate dehydrogenase was used for glutamine synthesis (see Krebs, 1935). However, the role of glutamate dehydrogenase in the metabolism of glutamate by guinea-pig and rabbit kidney slices has been questioned by and Preuss (1972), because these authors observed that glucose was formed from glutamate without concomitant accumulation of Vol. 188 ammonia; they concluded that the initial step in glutamate metabolism was transamination rather than oxidative deamination. So the pathways of renal metabolism of glutamate in these species remain uncertain.In an attempt to clarify this subject, we studied glutamate metabolism in isolated guinea-pig kidney tubules. The data obtained support the view of Krebs (1935) that glutamate dehydrogenase plays a central role in the renal metabolism of glutamat...