Contractile responses were produced by local application of levarterenol (norepinephrine) to the pial vessels of the mouse. The response was blocked by locally applied phentolamine mesylate. These data support the findings of workers who demonstrated that levarterenol diminishes cerebral blood volume in this species. The results are also in agreement with data from other species, which indicates that high doses of levarterenol are required to constrict pial arterioles. In the present study effective doses were in excess of 1μ/ml. These doses are probably higher than those present in blood or cerebrospinal fluid, either in normal or disease states. However, the local concentration of norepinephrine released by adrenergic nerves at the vessel wall remains unknown, as does the interaction of locally administered levarterenol with possible potentiating factors that are present in pathologic conditions.