Many investigators, including numerous studies from our laboratory, have used fluorescence microscopy to directly examine changes in permeability of pial vessels in rats (1–4), cats (5–7), mice (8,9), and hamsters (10–12) during basal and stimulated states. The use of cranial windows and fluorescence microscopy to examine the cerebral microcirculation has several advantages. First, the techniques allow the investigator to directly visualize cerebral microvessels. Thus, the investigator can visually examine changes in permeability of pial vessels during various stimuli and can visualize changes in reactivity of cerebral blood vessels during alterations in permeability of the blood-brain barrier (BBB). This allows the investigator to correlate changes in permeability of pial vessels with local cerebral hemodynamics. Second, the techniques allow the investigator to examine changes in permeability of pial vessels to differently sized and charged molecules. Thus, one can examine the effect of a specific stimuli on the selectivity of pial vessels. Third, the techniques allow the investigator to determine the precise sites, i.e., arteries, arterioles, capillaries, venules and/or veins, of disruption of the BBB during pathophysiologic conditions. This information may be of potential importance in the delivery of therapeutic agents to brain tissue during disease states. Fourth, the techniques allow the investigator to not only qualitatively, but quantitatively evaluate the permeability of pial vessels under basal conditions and during various conditions. With the calculation of clearance, the investigator can evaluate basal permeability of pial vessels and can determine the magnitude of disruption of the BBB in response to various stimuli.