Two key characteristics of the inflammatory response are the recruitment of leukocytes to inflamed tissue as well as changes in vessel permeability. We explored the relationship between these two processes using intravital confocal microscopy in cremasters of anesthetized (65 mg/kg Nembutal, ip) mice. We provide direct evidence that intercellular adhesion molecule-1 (ICAM-1) links leukocyte-EC interactions and changes in permeability (Ps). Importantly, we show that arterioles, not just venules, respond to proinflammatory stimuli, thus contributing to microvascular exchange. We identified two independent, ICAM-1-mediated pathways regulating Ps. Under control conditions in wild type (WT) mice there is a constitutive PKC-dependent pathway (Ps= 1.0±0.10 and 2.2±0.46 x10^-6 cm/sec in arterioles and venules, respectively), which was significantly reduced in ICAM-1 knockout (KO) mice (Ps= 0.54±0.07 and 0.77±0.11 x10^-6 cm/sec). The PKC inhibitor (Bisindolylmaleimid l, 1µmol/L in 0.01% DMSO) decreased Ps in WT mice to levels similar to those in ICAM-1KO mice. Likewise, a PKC activator (PMA, 1µmol/L in 0.01% DMSO), successfully restored Ps in ICAM-1KO vessels to be not different from WT controls. On the other hand, during TNF-induced inflammation, Ps in WT mice was significantly increased (2-fold in venules and 2.5-fold in arterioles) in a Src-dependent and PKC-independent manner. Blockade of Src (PP2, 2µmol/L in 0.01% DMSO) but not PKC, significantly reduced the TNF-dependent increase in Ps. We conclude that ICAM-1 plays an essential role in the regulation of Ps in microvessels, and that there are two separate (constitutive and inducible) signaling pathways that regulate permeability under normal and inflamed conditions.