Summary. Background: Previous studies have shown that ischemic preconditioning (PC) not only limits infarct size, but also improves arterial patency in models of recurrent thrombosis. We hypothesize that this enhanced patency is presumably because of a PC-induced attenuation of platelet-mediated thrombosis. However, there is, at present, no direct evidence that PC acts on the platelets per se and favorably down-regulates platelet reactivity. Objectives: Our goal was to test the concept that PC ischemia attenuates molecular indices of platelet activation-aggregation. Methods: Anesthetized dogs were randomly assigned to receive 10 min of PC ischemia followed by 10 min of reperfusion or a time-matched control period. Spontaneous recurrent coronary thrombosis was then initiated in all dogs by injury + stenosis of the left anterior descending coronary artery. Coronary flow was monitored for 3 h poststenosis, and molecular indices of platelet activation-aggregation were quantified by whole blood flow cytometry. Results: Coronary patency was, as expected, better-maintained following injury + stenosis in the PC group vs. controls (53% ± 5%* vs. 23% ± 5% of baseline flow, respectively; *P < 0.05). Moreover, PC was accompanied by: (i) a significant down-regulation of platelet-fibrinogen binding and formation of neutrophil-platelet aggregates (112% ± 14%* vs. 177% ± 21% and 107% ± 8%* vs. 155% ± 19% of baseline values in PC vs. control groups); and (ii) a trend towards a reduction in platelet P-selectin expression (148% ± 12%† vs. 190% ± 21% of baseline; *P < 0.05 and †P = 0.09 vs. control). Conclusion: These data provide novel, direct evidence in support of the concept that ischemic PC attenuates molecular indices of platelet activation-aggregation.