A voltage-jump technique for the study of the time course of small, voltage-induced changes in the structure of single bilayers is presented, and a method is introduced whereby electromechanical (electrostrictive) phenomena can be separated from dielectric relaxations. As no foreign molecules need be introduced into the bilayers, the question about probe artifacts is eliminated. The time constants and amplitudes of dielectric relaxations in oxidized cholesterol bilayers at 21 degrees C, along with their tentative identification are: (a) tau = 3.3 msec, deltaC/Co = 0.8% and tau = 0.7 msec, deltaC/Co = 0.6%: reorientation in the plane of the membrane of domains or clusters of dipoles. (b) tau = 155 musec, deltaC/Co = 1.5-3%: rotational reorientation of individual molecules. (c) tau = 18 musec, deltaC/Co - 1.4%: small amplitude reorientations individual dipoles about an axis lying in the plane of the membrane. Electrostictive effects with time constants between about 2 and 50 msec were also detected. A temperature study of both the dielectric and electrostrictive phenomena is reported. The application of the technique to other membrane compositions and to a variety of BLM problems is discussed.