We investigate microcirculation of the brain by studying changes in regional cerebral blood flow or in pial vascular diameters in the living mouse. We observe the effects of seizures on blood flow, and on vascular diameters and on the response of pial vessels to drugs. We observe the effects of agents related to mast cells and their contents. We observe the effects of environmental manipulation on the norepinephrine content of nerves to cerebral vessels and compare these effects with those produced on nerves to extracerebral blood vessels.