Increases in neural activity in the brain are followed by increases in blood flow and volume. While these changes in the flow of blood are commonly used as a surrogate for neural activity in fMRI and other imaging techniques, a detailed understanding of the underlying hemodynamic events has been lacking. One enduring controversy is what parts of the vascular network (arteries, capillaries or veins) account for the changes in blood volume during sensory stimulation. By using transcranial, two-photon laser scanning microscopy in awake mice, we were able to image spontaneous and sensory evoked changes in blood flow and volume at the level of single vessels. Sensory stimulation evoked large changes in arterial diameters, as well as slower, smaller changes in venous volumes. The changes in arteries and veins observed in awake animals were qualitatively different from those observed in anesthetized animals. These results provide a link between functional signals and their vascular origin.