The effect of Hg2+ and CH3-Hg+ on the passive and active transport properties of the Ca2+-Mg2+-ATPase-rich fraction of skeletal sarcoplasmic reticulum (SR) is reported. The agents abolish active transport, at 10(-5) and 10(-4) M concentrations, respectively. Addition of the mercurials was also shown to release actively accumulated Ca2+. The mercurials increase the passive Ca2+ and Mg2+ permeability in the absence of ATP at the same concentrations at which they inhibit transport. It is proposed that both effects are the result of direct binding of the mercurials to the SH groups of the Ca2+-Mg2+-ATPase pump, altering the conformational equilibria of the pump. The agents were also shown to increase the passive KCl permeability. The SR preparation consists of two vesicle populations with respect to K+ permeability, one with rapid KCl equilibration faciliated by a monovalent cation channel function and one with slow KCl equilibration. The mercurials increase the rates of KCl equilibration in both fractions, but produce higher rates in the fraction containing the channel function. The results are discussed in terms of pump and channel function and are compared with results for the electrical behavior of the CA2+-Mg2+-ATPase and other SR proteins in black lipid membranes, as presented by others.