Magnesium has built up the reputation of a 'natural calcium antagonist'. However, the exact effect of magnesium on coagulation and more specifically on platelet function is still disputed. An important discrepancy between in vivo and in vitro studies exists. Magnesium has thus been reported to antagonize platelets in some studies, and to stimulate platelets in other studies. Current evidence seems to point in the direction of a general antagonization of aggregation and coagulation.