Mg2+ and Ca2+ efflux rates from sarcoplasmic reticulum vesicles were measured by using a fluorescence chelate probe, chlortetracycline. Mg2+ efflux rate was almost the same as Ca2+ efflux rate under the same medium conditions. Both effluxes were activated by micromolar concentrations of extravesicular Ca2+ and blocked by millimolar concentrations of extravesicular Ca2+. Both effluxes were also activated by caffeine and inhibited by procaine. This result shows that Mg2+ can permeate through the Ca2+-induced Ca2+ release channel at the same rate as Ca2+: namely this channel has no selectivity between Ca2+ and Mg2+. Gating specificity of the channel was studied by measuring Mg2+ efflux. It was shown that this channel could be opened by Sr2+ as well as Ca2+, but not by Ba2+ or Mg2+. The apparent dissociation constant of the activation site for Sr2+ was slightly higher than that for Ca2+ but the apparent dissociation constant of the inhibition site for Sr2+ was almost the same as that for Ca2+.