As has been shown earlier, by applying external tensile stresses to a polymeric membrane it is possible for the membrane as a whole to be deformed elastically, while in areas around pores stress arises that is close to the yield stress of the polymer. This leads to a large plastic deformation and to the growth of pores. Experimental investigation of the loading of polyethylene terephthalate nuclear membranes by excessive gas pressure has confirmed this assumption. Control of changes in the porous structure of membranes was carried out both according to the gas-dynamic pore radius and by the conventional Knudsen equation. It was found that an excess of pressure over some limiting value causes irreversible growth of the pores. The stabilization time of the porous structure (at room temperature) during loading was ca. 10 min. Investigations of changes in porous structure of nuclear membranes under load have been carried out over a wide range of pore size. The growth ratio of pores reached several hundred percent depending on the initial pore sizes and free surface of the membrane.