Targeting mutant IDH, which is present in ~80% of glioma, is being tested as a new therapeutic approach. In the current study, we investigated metabolic alterations in response to mutant IDH inhibition by either AG-881 or BAY-1436032 in orthotopic patient-derived glioma models. Using high resolution in vivo 1H MRS we detected, in addition to a decrease in 2HG, an early increase in glutamate and the combined glutamine/glutamate signals. These were associated with slowdown of tumor growth and ultimately longer animal survival. This identifies potential early metabolic biomarkers of glioma response to mutant IDH inhibition.