Transcranial direct current stimulation (tDCS) is a noninvasive brain stimulation technique that induces changes in cortical excitability: anodal stimulation increases while cathodal stimulation reduces excitability. Imaging studies performed after unilateral stimulation have shown conflicting results regarding the effects of tDCS on surrogate markers of neuronal activity. The aim of this study was to directly measure these effects on activation-induced changes in regional cerebral blood flow (DrCBF) using positron emission tomography (PET) during bilateral tDCS. Nine healthy subjects underwent repeated rCBF measurements with 15 O-water and PET during a simple motor task while receiving tDCS or sham stimulation over the primary motor cortex (M1). Motor evoked potentials (MEPs) were also assessed before and after real and sham stimulation. During tDCS with active movement, DrCBF in M1 was significantly lower on the cathodal than the anodal side when compared with sham stimulation. This decrease in DrCBF was accompanied by a decrease in MEP amplitude on the cathodal side. No effect was observed on resting or activated rCBF relative to sham stimulation. We thus conclude that it is the interaction of cathodal tDCS with activation-induced DrCBF rather than the effect on resting or activated rCBF itself which constitutes the physiological imaging correlate of tDCS.