Leucocyte-endothelial cell adhesion is modulated by a variety of adhesion glycoprotein expressed on the surface of leucocytes and endothelial cells. Although in vitro studies show that these adhesion molecules mediate the decrease in leucocyte rolling velocity and the increase in leucocyte adherence and emigration associated with inflammation, there are few in vivo data to support this hypothesis. The aim of this study was to assess the role of leucocyte (CD11b/CD18) and endothelial cell (P- and E-selectin) adhesion molecules in mediating the leucocyte-endothelial cell adhesion elicited in rat mesenteric venules during a model of longlasting intestinal inflammation. Indomethacin was injected 48 and 24 hours before the experiment. The mesenteric microcirculation was observed by intravital microscopy in animals treated with monoclonal antibodies (MAb) directed against either P-selectin, E-selectin, or CD11b/CD18. Leucocyte rolling velocity, and the number of adherent and emigrated leucocytes as well as vessel diameter and erythrocyte velocity were monitored in roughly 30 micron diameter postcapillary venules. Indomethacin treatment resulted in mucosal ulceration and granulocyte infiltration, and a corresponding inflammatory response in the mesentery, which was characterised by an increase in the number of adherent (eightfold) and emigrated (sixfold) leucocytes and a reduction (80%) in leucocyte rolling velocity. The indomethacin induced leucocyte-endothelial cell adhesion in mesenteric venules was significantly reduced by treatment with MAbs against either CD11b/CD18 or E-selectin, but not by the P-selectin MAb. These results suggest that both leukocyte (CD11b/CD18) and endothelial cell (E-selectin) adhesion molecules contribute to the granulocyte accumulation in a chronic model of intestinal inflammation.