88Studies of the structure and function of energy-conserving membranes during the bacterial cell cycle are frustrated by methods of cell cycle analysis. Such methods are reviewed and their suitability for such studies discussed. Respiration rates of synchronous cultures generally exhibit marked discontinuities increasing in an oscillatory or stepwise fashion. The pattern of respiratory development may, in part, be influenced by the nature of the growth medium, but the molecular mechanism(s) of control remain to be identified. The possibility of control by adenylate pools is discussed. Proteins and lipids may be synthesized and incorporated into the membrane asynchronously so that fluctuations in the protein to lipid ratio of membranes occur during the cycle. Such fluctuations result in changing membrane density and fluidity, and may thus modulate the activity of membrane-bound enzymes. Activities of succinate, NADH dehydrogenases, and of adenosine triphosphate, and the amount of cytochrome b, all exhibit discontinuous increases in the cell cycles of various bacteria. Again, underlying control mechanisms remain obscure. Alternative models for the spatiotemporal growth of membranes, and their consequences for the segregation at cell division of respiratory chain components, are described. Localization of membrane growth, together with conservation of newly and previously synthesized membrane zones, is proposed to account for the observed unequal segregation of nitrate reductase and cytochromes at cell division.