1. The metabolic situation found in pancreatic islets exposed to both L-glutamine and 2-ketoisocaproate was investigated in order to assess its relevance to the synergistic effects of these nutrients upon insulin release. 2. In islet homogenates, serveral 2-keto acids could be used as partners for the transamination of L-glutamate to 2-ketoglutarate. The rate of transamination did not correlate positively with the capacity of each 2-keto acid to stimulate insulin release in the presence of L-glutamine. 3. L-Glutamine enhanced the production of L-leucine from 2-ketoisocaproate and inhibited the conversion of the 2-keto acid to acetoacetate and CO2. L-Glutamine also inhibited the oxidation of pyruvate. 4. In the presence of 2-ketoisocaproate, the rate of generation of 2-ketoglutarate from exogenous L-glutamine was increased, but the oxidative deamination of glutamate was suppressed. 5. L-Valine antagonized the effect of 2-ketoisocaproate to augment 14CO2 output from islets prelabelled with L-[U-14C]glutamine. 6. L-Glutamine did not increase the islet content of reduced pyridine nucleotides beyond the high level reached in the sole presence of 2-ketoisocaproate. 7. If allowance was made for the influence of exogenous nutrients upon the oxidation of endogenous nutrients, the insulin output evoked by L-glutamine and/or 2-keto acids tightly depended on the increment in oxidation rate attributable to these nutrients. 8. The metabolic and secretory responses to L-glutamine and 2-ketoisocaproate were best explained by a stimulation of transamination reactions between 2-ketoisocaproate and glutamate derived from exogenous glutamine.