A general feature that appears to be emerging for the structure of biological membranes is the existence of an asymmetrical phospholipid distribution between the two membrane surfaces. Of fundamental importance to the discovery of such asymmetry are the questions regarding its assembly and maintenance. We propose to examine these processes by investigating the transmembrane exchange or flip-flop and transmembrane movement of phospholipids in rat liver microsomes. Furthermore, we wish to study the possibility of these lipid motions occurring in bilayer vesicles to ascertain whether it is necessary to invoke the presence of specific lipid translocating proteins in biological membranes.