We report on a strategy for the "turn-on" detection of target biochemical (metabolic) reactions using a triple resonance NMR technique with an isotope-labeled probe. Our NMR study clearly reveals that otherwise NMR-nonactive-C-13/H-2-labeled glucose actually turns "on" its H-1 NMR signal by conversion to an important biomarker lactate as the end product of anaerobic glycolysis in cells and in injected mice with high selectivity.