Three modifications of a stationary membranous oxygenator (SMO) with utilization of new selective compositional membranes were developed. Test procedures for the SMO in the system gas-gas and gas-blood on a stand and in acute experiments on animals are described. The results obtained demonstrated that as concerns the transport of oxygen and carbon dioxide, as well as the stability in operation it is the model with the projections coincidently positioned on the upper and lower membranes whereby the peripheral blood is fed most efficiently.