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BACKGROUND: Vascular complications and abnormal platelet function contribute to morbidity and mortality in diabetes mellitus. We hypothesized that the Rho-related GTPase protein, Rac1, can influence both endothelial and platelet function and might represent a potential novel therapeutic target in diabetes mellitus. METHODS AND RESULTS: We used both in vitro and ex vivo approaches to test the effects of pharmacological inhibition of Rac1 during hyperglycemic condition. We evaluated the effect of NSC23766, a pharmacological inhibitor of Rac1, on vascular function in diabetic mice and platelet aggregation in diabetic subjects. We demonstrated that the administration of NSC23766 protects from hyperglycemia-induced endothelial dysfunction, restoring NO levels, and reduces oxidative stress generated by nicotinamide adenine dinucleotide phosphate oxidase. Mechanistically, we identified Rho-associated coiled-coil serine/threonine kinase-1 as a downstream target of Rac1. Moreover, we reported that during hyperglycemic conditions, human platelets showed hyperactivation of Rac1 and impaired NO release, which were both partially restored after NSC23766 treatment. Finally, we characterized the antiplatelet effect of NSC23766 during hyperglycemic conditions, demonstrating the additional role of Rac1 inhibition in reducing platelet aggregation in diabetic patients treated with common antiplatelet drugs. CONCLUSIONS: Our data suggest that the pharmacological inhibition of Rac1 could represent a novel therapeutic strategy to reduce endothelial dysfunction and platelet hyperaggregation in diabetes mellitus.