The relationship between acute endothelial dysfunction and the extravasation of leukocytes was studied in vivo with intravital microscopy of the rat mesenteric microvasculature. Acute endothelial dysfunction of the rat mesenteric microvasculature was induced in vivo by superfusing the mesentery for 90 min with one of three different stimulating agents: NG-nitro-L-arginine methyl ester (L-NAME, 50 microM), thrombin (0.5 U/mL), or hydrogen peroxide (H2O2, 50 microM). All three agents induced a similar increase in leukocyte rolling and adherence, which was significantly greater than that observed in control rats superfused with Krebs-Henseleit solution (P < 0.01). Transendothelial migration of leukocytes into the perivascular space was also increased by superfusion with L-NAME, thrombin, or H2O2. However, there was a greater increase in the number of migrated leukocytes in the rat mesentery after L-NAME and H2O2 superfusion than that observed during thrombin superfusion. In vivo infusion of a neutralizing antibody against platelet-endothelial cell adhesion molecule-1 (PECAM-1) specifically inhibited L-NAME-induced and H2O2-induced migration of leukocytes but did not prevent extravasation of leukocytes induced by thrombin. In rat mesenteries superfused with the three different stimuli, immunohistochemical analysis of endothelial cell adhesion molecules expressed on the microvascular endothelium revealed a significant increase of ICAM-1, but not PECAM-1, endothelial cell surface expression (P < 0.01 and P > 0.05 vs. control rats, respectively). Our data confirm a key role for PECAM-1 acutely in leukocyte extravasation in vivo and indicate that the involvement of constitutively expressed PECAM-1 in leukocyte transendothelial migration is preferentially correlated to oxidative stress-related stimuli in the microvascular endothelium.