In this study, we tested the hypothesis that reducing shear rates in postcapillary venules causes CD18-dependent, selectin-independent leukocyte rolling. Intravital microscopy was used to assess shear rate-dependent leukocyte rolling in 25- to 40-microns rat mesenteric venules. Pretreatment of animals with 25 mg/kg fucoidin, a carbohydrate moiety that binds to and inhibits selectin function, essentially abolished the number of spontaneously rolling leukocytes. When shear rates were reduced by 50% (from 438 +/- 36 s-1 to 222 +/- 19 s-1) in the presence of fucoidin, leukocyte rolling increased fourfold, suggesting a selectin-independent mechanism of leukocyte rolling. Administration of CL26, an anti-CD18 antibody, prevented the leukocyte rolling associated with reduced shear rates. A second objective was to determine if the integrin-mediated leukocyte rolling at reduced shear rates would lead to firm adhesion of leukocytes in the presence of a chemotactic stimulus. Animals were pretreated with fucoidin and 100 nmol/L platelet-activating factor (PAF) was superfused over the mesentery. Fucoidin prevented leukocyte rolling and subsequent PAF-induced adhesion at normal shear rates; however, when shear rates were reduced by 50%, a significant CD18-dependent increase in leukocyte rolling (10-fold) and adhesion (5-fold) was noted within 15 minutes. These data raise the possibility that, at lower shear rates, as is the case in various inflammatory conditions, selectin-independent, CD18-dependent leukocyte rolling and subsequent adhesion can occur in postcapillary venules.