Our recent study (Saiki, Y., and Ikemoto, N., Biochemistry 38, 3112-3119, 1999) suggests that Ca2+ release and re-uptake of the released Ca2+ are coordinated. The following results suggest that the coordination is mediated by the luminal Ca2+ ([Ca2+](lum)) transient, Upon inducing the release of the passively loaded Ca2+ from the SR with polylysine, the luminal Ca2+ ([Ca2+](lum)) fist increased then decreased ([Ca2+](lum) transient). The activity of the SR Ca2+ ATPase was monitored at different times after inducing Ca2+ release, The phosphoenzyme (EP) formation as determined by the MANT-fluorescence increased concurrently with the initial rapid increase in the [Ca2+](lum). EP decay (pumping turnover) was accelerated concurrently with a decrease of the [Ca2+],,. The results suggest that the [Ca2+](lum) transient serves as a mediator for the acceleration of the Ca2+ re-uptake occurring soon after the induction of Ca2+ release.