The ultrastructural changes of the platelets stimulated by release inducers II such as thrombin, collagen, and latex particles were studied to compare the differences in morphology of release I and II. The characteristics of platelets in response to thrombin were the decrease in number of alpha-granules and marked dilation of the open canalicular system containing fibrin preceded by the disappearance of very dense granules. In the platelets in response to collagen or latex, the release of alpha-granules was initiated after taking the particles into the open canalicular system. In all of the cases, the amount of microfibrils was seen in the cytoplasm, while contraction wave of the microtubules was rare in the cytoplasm during the release and uptake of particles. Accordingly, the release of very dense granules seems to be followed by that of alpha-granules. It is suggested that the microfibrils play a more important role than the microtubules in the release II as well as release I. Furthermore, the contents of alpha-granules may be discharged via the open canalicular system on the release II by thrombin, collagen or latex particles.