The destructive effect of heat on platelet function was established by early investigators. Warming platelets to temperatures above 42 C. was found to destroy their clot-retracting capacity. In the present study, electron microscopic observation has been correlated with tests of platelet function to determine the basis of the damaging effect of heat on platelet physiology. Platelets exposed to temperatures above 42 C. for brief periods lose their discoid shape and become irregularly swollen. The granules of heated platelets are transported to the cell centers where they fuse together. Heat-altered platelets do not aggregate in response to ADP or thrombin and will not retract clots.