The author considers the aspects of the mechanism and conditions of formation of pinocytosis vesicles in the capillary endothelium as a function of the size of the protein cluster formed from plasma proteins on the surface of the plasma membrane of the endotheliocyte and initiating the formation of a caveola and the value of the intracapillary hydrostatic pressure ensuring the energetics of the transition of the caveola to a vesicle and its detachment. The boundary parameters of the formation of vesicles are calculated and cmopared with known experimental data. It is assumed that in other forms of endocytosis, too, the iniital phase of the process (formation of a caveola) is associated with the adsorption of the protein molecules on the plasma membrane and their clustering. A possible explanation of the increased number of vesicles in the endothelium in hypertension is offered. © 1989.