The morphology of the three types of endothelial vesicles in fenestrated and non-fenestrated capillaries from various sources (human skin, senile dermal angiomas, frog tongue and rat renal medulla) has been studied. The micropinocytotic vesicles of Palade were prevalent in the non-fenestrated endothelium with most caveolae closed by either amorphous material of low electron density or a caveolar membrane. The initial stage of the opening of vesicles onto the surface plasma membrane and the terminal stages of separation were indefinite. Those vesicles which were fusing with or separating from adjacent vesicles displayed amorphous material or an intervesicular membrane at the line of junction. Such features were present irrespective of the relationship of the vesicles to the plasma membrane and of the fenestration of the endothelium. The caveolar and intervesicular membranes vary in morphology as do those bridging the conventional fenestrae.