Local shear stresses generated by blood flow exert direct mechanical effects on adhesion of circulating leukocytes to vascular endothelium, but their effects on expression of endothelial-leukocyte adhesion molecules have not been determined. Shear stress in rabbit carotid arteries was increased by 170% or decreased by 73% in 5 days by surgical manipulations. En face immunofluorescence staining with the monoclonal antibody Rb1/9 revealed that vascular cell adhesion molecule–1 (VCAM-1) expression was greatly increased under low shear stress, but the distribution of staining was patchy. Thus, 71.4±7.8% of fields were VCAM-1 positive versus 2.4±0.47% of fields in control arteries. Frequently, large regions showed consistent but heterogeneous staining. Occasionally, small islands of cells were labeled intensely. Monocytes, detected by use of the monocyte-specific antibody HAM 56, adhered to endothelium under low shear stress; 64.5±8.2% of the monocytes colocalized with detectable VCAM-1, although many (83.2±2.8%) VCAM-1–positive regions were devoid of monocytes. VCAM-1 expression also increased significantly but to a lesser extent when shear stress was approximately doubled. Thus, 8.7±1.5% of fields were VCAM-1 positive under high shear versus 2.5±0.87% under normal shear stress. No monocytes were detected at high shear stress. At normal shear stresses, intercellular adhesion molecule–1 (ICAM-1), detected by use of the monoclonal antibody Rb2/3, was extensively distributed; thus, 53.5±5.5% of fields contained ICAM-1–positive cells. The junctional regions of the cells were heavily stained. Experimental increases in shear stress significantly upregulated ICAM-1 expression (88.3±2.0% of fields were ICAM-1 positive; P<.05). Staining was again concentrated in the cell junctional regions. Reduced shear stress suppressed ICAM-1 expression (16.6±10.3% of fields were positive), and ICAM-1 was more commonly, but not exclusively, distributed diffusely. Junctional ICAM-1 may participate in endothelial cell-cell adhesion; alternatively, a pool of ICAM-1 concentrated between cell junctions may be inaccessible to circulating leukocytes until endothelial cell activation presents the molecule to the vessel lumen.