Rapid mixing technique was used to evaluate the kinetic properties of the Ca2+ pump in cardiac SR isolated from young and old rat hearts. Significant age-related differences in the initial rates of ATP-dependent 45Ca2+ uptake were not served, indicating that the intrinsic properties of the Ca2+ pump are not altered during aging. This suggests that the prolonged contraction duration interval in senescent myocardium may be the result of a decrease in SR content rather than a decline in Ca2+ pump turnover rate. A detailed comparison of the mammalian Na,K-ATPase partial reactions with the transient electrical currents generated by the Na+ pump in response to an ATP concentration jump has demonstrated that the Na+ translocation mechanism is a two step process, consisting of a rapid conformational change followed by a slower electrogenic Na+ deocclusion reaction. Presteady state-kinetic studies on Na+-H+-exchanger in renal brush border membrane vesicles have began to uncover details of the temporal relationship between Na+ uptake and H+ efflux. A new method has been developed for incorporating long chain fluorescent phospholipids into plasma membranes and following their distribution in membrane structural domains.