Shear stress encountered in stenosed human arteries is able to induce a certain range of platelet activation. In order to determine the extent of platelet shape change induced by high shear rate conditions, we used electron microscopy (EM) and immuno-EM to study platelet ultrastructure from blood flowing in vivo through stenosed arteries. Then it was compared with platelets from healthy controls exposed in vitro to a shear rate of 4000 s(-1). Six patients with stenosed arteries (iliac, femoral and renal) were investigated at the time of transcutaneous angiography. Blood was harvested from the same catheter in the stenosed artery and in the abdominal aortic artery (control sample), each patient being its own control. The percentage of platelets with shape changes (loss of discoid form, pseudopod emission, organelle centralisation) significantly increased in samples from stenosed arteries. Shape change was concomitant with the membrane glycoprotein IIb-IIIa distribution at the pseudopod extremities. These activated platelets had not completed secretion and were maintained in a reversible activation state. Similar results were obtained on platelets from healthy donors submitted in vitro to a high shear rate. In conclusion, this study shows that the high shear rate encountered in human stenosed arteries is able to induce shape change and reversible activation of platelets in vivo.