(1) Ca2+ efflux from rabbit skeletal muscle sarcoplasmic reticulum vesicles pre-loaded with 45Ca2+ was studied in the presence and in the absence of external Ca2+. (2) In the absence of Ca2+ in the assay media, ADP activates the Ca2+ efflux. The increment of Ca2+ efflux requires Pi, is coupled to ATP synthesis, and is inhibited by external Ca2+ (Ki 0.1-0.2 muM). (3) When Ca2+ is added to the assay media, ADP alone activates the Ca2+ efflux, but this is coupled to a Ca2+ influx of the same magnitude. It is therefore an exchange of internal for external Ca2+ in a 1:1 ratio. (4) The ADP-activated Ca2+ exchange requires external Ca2+ with an apparent Km of 0.1-0.2 muM, does not require the addition of Pi or Mg2+, although 3-10 mM MgCl2 activates it. It is not inhibited by the removal of contaminating ATP with hexokinase plus glucose. (5) It seems likely that Ca2+ can be translocated across sarcoplasmic reticulum membrane without the formation of a phosphorylated intermediate.