Low-grade glioma (LLG) patients have a relatively long-term survival of ~13 years but these tumors always recur. Since IDH mutations are present in >80% of LGGs, inhibition of mutant IDH activity is being tested as a new therapeutic approach. Here, we investigated response to mutant IDH inhibition by AG-881 in orthotopic LGG mouse models. Using in vivo 1H MRS we detected, in addition to a decrease in 2-HG, an early increase in both glutamate and glutamine/glutamate that were associated with subsequent tumor shrinkage. This identifies potential early metabolic biomarkers of LGG response to mutant IDH inhibition.