The temperature dependence of passive Ca2+ efflux from skeletal muscle fragmented sarcoplasmic reticulum (FSR) was studied by dilution of a suspension of the vesicles into which 1 mM (CaCl2 + 45Ca) had been passively incorporated by overnight incubation at 3 degrees. It was found that in the presence of 5 mM Mg2+, Ca2+ efflux could be resolved into two simultaneous first-order processes between 5 degrees and 35 degrees, but only a single first-order process appeared between 37 degrees and 55 degrees. Two independent functional transitions were found at 30 degrees, indicating an abrupt membrane molecular reorganization at that temperature: (1) The two components of Ca2+ efflux at 5 degrees--35 degrees contributed equally to the total observed initial efflux at temperatures up to 30 degrees. Between 30 degrees and 35 degrees, the relative contribution of the fast component progressively diminished until, by 37 degrees, only the slow component remained. (2) The slow component, which persisted throughout the entire temperature range 5 degrees--55 degrees, exhibited a break in its Arrhenius plot at 30 degrees--32 degrees. Elevation of internal Ca2+ concentration to 10 mM failed either to produce saturation kinetics of efflux or appreciably change its first-order rate constant. Omitting Mg2+ in the low temperature range accelerated Ca2+ efflux about 20-fold and eliminated the fast component, whereas including Ca2+ in the external medium in the high temperature range retarded Ca2+ efflux by about the same factor and generated a fast component. Omitting Mg2+ in the high-temperature range, however, had little effect on Ca2+ efflux. The failure of external divalent cation to stimulate Ca2+ efflux thus precludes an obligatory carrier-mediated exchange mechanism. Furthermore, participation of the catalytic turnover function of the Ca2+-ATPase molecule in Ca2+ efflux was unlikely because (1) the 30 degrees transition temperature for efflux did not coincide with those previously determined for active Ca2+ uptake, ATPase activity, and reversal of the Ca2+ pump, and (2) above the transition temperature, the activation enthalpy and activation entropy increased for efflux but decreased for both active Ca2+ uptake and ATPase activity. Ca2+ efflux therefore probably involved simple diffusion through a membrane pore (Ca2+ "leak"). By comparison to the results of others using artificial and biological membranes, the effect of external divalent cation to produce a fast component of Ca2+ efflux from FSR is tentatively attributed to the formation of aggregates of SR vesicles.