In our early work in developing activated clotting time (ACT) assays, it became apparent that changes occurred in coagulation times as a whole blood sample aged (0-6 h). Subsequent studies showed that the coagulation parameters of plasma obtained from the samples remained stable during this time frame. These changes in whole blood clotting times during storage were eventually traced to the platelets. Several years of work demonstrated that this change was due to the removal of the blood from the vascular lining. This recalled a mechanism that was originally put forth in the 1970s with the discovery of prostacyclin. In this postulated mechanism, platelets are ‘time-bombs’. They are kept under control by prostacyclin (PGI2) secreted by the vascular lining. Without this prostacyclin, platelets ‘preactivate’. Since that time, additional substances secreted by the vascular endothelium have been identified, such as nitric oxide, that also influence platelet activity. The ‘preactivation’ of platelets in a blood sample can be followed using an ACT. In the same donor, the preactivation is uniform and reproducible over an extended period (months). There is, however, considerable variability between donors. Some donors’ platelets preactivate dramatically, while other donors show hardly any change. Prostacyclin, added to the blood sample when it is collected, prevents this preactivation. The clinical significance of these observations has yet to be clearly established, but these observations raise a number of questions with respect to methods for improving platelet function during bypass and in evaluating the risk of platelet-mediated cardiovascular disease.