Exogenous L-glutamine is actively metabolized in rat pancreatic islets. The rate of L-glutamine deamidation largely exceeds the rate of glutamate conversion to gamma-aminobutyrate and alpha-ketoglutarate. The latter conversion occurs in part by oxidative deamination, and in part by transamination reactions coupled with the conversion of 2-keto acids (pyruvate, oxaloacetate), themselves derived from the metabolism of glutamine, to their corresponding amino acids (alanine, aspartate). An important fraction of malate formed from alpha-ketoglutarate leaves the Krebs cycle and is converted to pyruvate, the process being apparently associated with the induction of a more reduced state in cytosolic redox couples. L-Glutamine abolishes the oxidation of endogenous nutrients is documented by the fact that the glutamine-induced increase in O2 consumption is much lower than expected from the rate of 14CO2 output from islets exposed to L-[U-14C]glutamine, L-Glutamine, although decreasing K+ conductance, fails to stimulate insulin release both in the absence and presence of D-glucose. It is proposed that L-glutamine represents a major fuel for pancreatic islets under physiological conditions.