15 min of strenuous exercise caused a highly significant rise in the venous platelet count without altering platelet adhesion to glass. It also produced a significant increase in the rate of the second phase of ADPand adrenaline-induced aggregation, 5-HT-induced aggregation and disaggregation, and collagen-induced aggregation. These changes in platelet number and function following exercise should be taken into account when anti-platelet drugs are assessed in human subjects, and when changes in these parameters are sought in disease states.