The underlying biological mechanisms of transcranial direct current stimulation (tDCS) remain relatively uncharacterized. In this report, MRI measures for cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO2) of healthy controls was acquired before and during tDCS to better understand real-time neuronal response. We observed a significant increase in global CBF suggesting an increase of neuronal activity during compared to before tDCS. CMRO2 showed an increase of ~7% during tDCS compared to before tDCS. We suggest that the reported cortical excitability increase may also serve as a potential biomarker for neuronal reactivity or plasticity in many neurodegenerative diseases.