Fluidity as a prominent feature of the phospholipid portion of biological membranes, as well as of model phospholipid bilayer systems, has been detected by numerous physical techniquesl). However, correlation of this fluidity with biological functions of membranes is, as yet, documented in only a few cases. For example, fatty acid auxotrophs of E. coli grown on different fatty acids exhibit an abruptly increased rate of transport of metabolites across the cell wall at temperatures above the "melting" temperature of the fatty acid supplement e,:~). The physical properties of lipids extracted from E. coli also reflect the temperature at which the bacteria were grown 4). Fluidity of hydrocarbon chains has been related to the calcium dependent ATPase activity of sarcoplasmic vesiclesS). A number of other essential functions of biological membranes may very well be associated with flu-idity6,7), but such considerations are limited by lack of precise knowledge of the molecular basis of fluidity and of the rates of motions involved. The following discussion will review the use of spin labels 8-1~) to determine the rates of several of the motions involved in the fluidity of phospholipid bilayers and, where possible, to provide a structural basis for these motions.