An intravital microscope (IV 500; Mikron Instruments, San Diego, CA) equipped with water immersion objectives (Carl Zeiss, Inc., Thornwood, NY) was used in experiments. Small boluses (∼50 μl) of BCECF-labeled cells were injected through the right carotid artery catheter. Fluorescent cells were visualized in the left frontoparietal skull by video-triggered stroboscopic epi-illumination (Chadwick Helmuth, El Monte, CA) through an FITC filter set and an ×10 objective (Zeiss Achroplan, numerical aperture [NA] 0.3 ∞, Water). 150 kD of FITC dextran (Sigma Chemical Co.) was injected in some experiments for measurements of microvascular dimensions using an ×40 objective (Zeiss Achroplan, NA 0.75 ∞, Water) as previously described (17 (link)). At the end of some experiments, saline containing 1 mg/ml rhodamine 6G or 4 mg/ml rhodamine 123 (Molecular Probes) was injected intravenously at a dose of 1.5 ml/kg body weight. The distribution of FITC-dextran (Sigma Chemical Co.) and rhodamine compounds in the skull was recorded through an ×4 objective (Achroplan, NA 0.16). All scenes were recorded on video tape using a SIT camera (VE 1000-SIT; Dage MTI, Michigan City, IN), a time base generator (For - A, Montvale, NJ), and a Hi-8 VCR (Sony, Boston, MA).