Blood flowing through artificial organs or arterial stenoses is subjected to high shear for short times. To clarify the effects of short acting high shear forces upon platelets, heparinized PRP was exposed to viscometric flow (57-255 N/m2; 7-700 ms). Before and after shear exposure beta-TG release, LDH liberation, platelet count, and ADP-induced aggregation were assayed. Stypven time-monitored platelet procoagulant activity, determined after heparin neutralization by protamine, proved to be the most sensitive indicator for shear-induced platelet alteration. A shear stress of 170 N/m2, acting for as short a time as 7 ms made available procoagulant phospholipids, whereas beta-TG and LDH liberation required 255 N/m2 for 7 ms, and ADP refractoriness was found only after 113 ms exposure to 255 N/m2. Under these conditions the percent release of beta-TG does not exceed the liberation of LDH, suggesting that the evidence for "shear induced platelet activation" from experiments with exposure times of minutes, is most likely a conventional biochemical rather than biophysical effect.