Because thrombus formation at the site of endothelial injury has been thought to be a critical step in the pathogenesis of bacterial endocarditis, the effect of aspirin on experimental valvular thrombosis and bacterial endocarditis in rabbits was evaluated. Aortic valvular injury and thrombosis were induced in aspirin-treated and control rabbits with intracardiac catheters. A subsequent inoculation of Streptococcus viridans resulted in the development of infective endocarditis. Rabbits were sacrificed as early as 6 hr, and the effectiveness of aspirin was determined by the weight of the sterile vegetations and the quantitation of bacteria in the thrombotic vegetation. Aspirin, in levels in excess of 50 mg/dl did not attenuate the evolution of infective endocarditis, since the formation of sterile thrombotic vegetation and bacterial endocarditis in aspirin-treated rabbits was similar to those in controls.