Platelet activation is accompanied by distinct morphological changes from disc-shaped cells to more rounded particles with multiple blebs or psuedopodia. A quenched-flow approach has been used to follow the kinetics and nature of these morphological events. Scanning-electron micrographs revealed very rapid alterations in platelet shape. At 0.5 S after activation with ADP or thrombin, the number of resting disc-shaped particles was nearly halved and short blebbed forms were maximal at 0.5 S. By 1.7 S about 60% of particles were in fully activated or multiple-blebbed form. The calcium ionophore A-23187 caused slightly slower effects. Adrenalin was much less potent, with about 14% of platelets becoming fully activated by 10 S. Control experiments showed only small changes in particle morphology when unactivated platelets were pumped through the reaction tubing and then quenched in glutaraldehyde. The resistive volume of platelets increased by 0.42 fl at 0.5 S after ADP stimulation and was essentially independent of variations in particle shape. These results show that the quenched-flow approach can provide new information about platelet function and that morphological changes begin earlier than previously thought.