Abnormal adhesion of sickle red blood cells (SS RBCs) to vascular endothelium may play an important role in vaso-occlusion in sickle cell disease (SCD). Accruing evidence shows that endothelial V3 integrin has an important role in SS RBC adhesion because of its ability to bind several adhesive proteins implicated in this interaction. In the present studies, we tested therapeutic efficacy of small-molecule cyclic pentapeptides for their ability to block V3-mediated SS RBC adhesion by using two well-established assay systems, i.e., cultured human umblical vein endothelial cells (HUVEC) and artificially perfused mesocecum vasculature of the rat under flow conditions. We tested the efficacy of two RGD-containing cyclic pentapetides, i.e., cRGDFV (EMD 66203) and cRGDF-ACHA (-amino cyclohexyl carboxylic acid) (EMD 270179), based on their known ability to bind V3. An inactive peptide, EMD 135981 (cR-ADFV) was used as control. Cyclization and the introduction of D-Phe (F) results in a marked increase in the ability of cyclic peptides to selectively bind V3 receptors. In the mesocecum vasculature, both EMD 66203 and EMD 270179 ameliorated PAF-induced enhanced SS RBC adhesion, postcapillary blockage and significantly improved hemodynamic behavior. Infusion of a fluorescent-derivative of EMD 66203 resulted in co-localization of the antagonist with vascular endothelium. Also, pretreatment of HUVEC with either V3 antagonist resulted in a significant decrease in SS RBC adhesion. Because of their metabolic stability, the use of these cyclic V3 antagonists may constitute a novel therapeutic strategy to block SS RBC adhesion and associated vaso-occlusion under flow conditions.