The sensor for blood-flow need with neural activity and exercise is not known. We tested the hypothesis that accumulation of electrons in cytosolic free nicotinamide adenine dinucleotide (NAD) activates redox signaling pathways to augment blood flow. NAD is the primary carrier of electrons from glucose and lactate for ATP synthesis. Because increased glycolysis transfers electrons from glucose to NAD+ faster than they are used for mitochondrial ATP synthesis, electrons accumulate in cytosolic NADH. Because cytosolic NADH and intra- and extracellular lactate/pyruvate (L/P) ratios are all in near-equilibrium, NADH can be increased or decreased by i.v. lactate or pyruvate. Here, we report that elevated plasma LJ`P in non-nal rats increases blood flow in numerous resting tissues and augments blood flow increases in activated somatosensory (barrel) cortex and contracting skeletal muscle. Increased flows are largely prevented by injection of pyruvate (to lower L/P), a superoxide dismutase mimic (to block vascular effects of superoxide), or an inhibitor of nitric oxide synthase (to block *NO vasodilatation). Electrons carried by. NADH, in addition to fueling ATP synthesis, also fuel redox signaling pathways to augment blood flow in resting and working tissues. These novel findings are fundamental to understanding blood-flow physiology and pathology.