Abstract RG13965, a pseudotetrapeptide analogue of Arg-Gly-Asp (RGD), inhibited collagen-induced dog, monkey, human, hamster, mouse, and pig platelet aggregation in vitro with IC 50 values of 3.7, 4.6, 6.3, 126, 136 and 1600 μM, respectively. RG13965 (3, 10, and 30 mg/kg, i.v.) decreased the incidence of collagen/epinephrine-induced thrombosis in mice from 90% in untreated animals to 63, 37, and 0%, respectively. In hamsters, RG13965 (10 and 30 mg/kg, i.v.) prolonged the time required for formation of a hemostatic plug in severed mesenteric arteries by 1.6- and 3.6-fold, respectively. In a canine model of repetitive platelet thrombus formation in the coronary artery, RG13965 (0.1, 0.3, and 1 mg/kg, i.v.) reversibly inhibited cyclic flow reductions (CFRs) and inhibited ADP-induced ex vivo platelet aggregation by 29, 57, and 77%, respectively. RG13965 (1 mg/kg) completely inhibited CFRs for at least 40 min. Platelet count was not altered at any dose and template bleeding time was prolonged modestly (1.8-fold) at only the highest dose. RG13965 dose-dependently and reversibly inhibited thrombus formation at doses which did not completely inhibit ex vivo platelet aggregation and only modestly prolonged template bleeding time.