In vivo activation of platelets, produced by damaging an artery in the rat with ultrasound, was studied with the electron microscope. We performed both a three-dimensional reconstruction by thin serial sections and a morphometric study of the activation process. This is characterized by exocytosis of the content of granules, widening of tubules of the open canalicular system, and emission of pseudopodia. The three-dimensional reconstruction suggested that some extended pseudopodia adhere to the arterial intima, and confirmed former observations that a locomotor apparatus differentiates in platelets adhering to the arterial intima. We speculate that the contraction of some pseudopodia may pull the platelet body toward the arterial wall.The morphometric study revealed that during activation the volume of the open canalicular system volume increases and that of the dense granules decreases; all other compartments did not change.