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The ATPase activities and phosphoenzyme levels have been determined in sarcoplasmic reticulum (SR) membranes prepared from two animal models of muscular dystrophy, myodystrophic (myd/myd) and strain 129 dystrophic (129 dy/dy) mice. In both myd/myd and 129 dy/dy SR membranes, the basal ATPase activities are elevated above control levels, while the Ca-dependent ATPase activities are normal. The addition of 0.1% Triton X-100 not only lowers the basal ATPase activity of myodystrophic control SR membranes by 60%, but also lowers the elevated basal ATPase activity of myd/myd SR membranes to a similar level. The Ca-dependent ATPase activities of myodystrophic control and myd/myd SR membranes are increased approximately threefold by the addition of Triton. The addition of 0.1% Triton X-100 lowers the basal ATPase activities of 129 control and 129 dy/dy SR membranes to similar levels, but stimulates the CA-dependent ATPase activity of 129 dy/dy SR membranes to a level that is only 60% of that of 129 control SR membranes. The level of phosphoenzyme intermediate is decreased approximately 15% in myd/myd SR membranes and approximately 30% in 129 dy/dy SR membranes.