During the stepwise solubilization of sarcoplasmic reticulum vesicles with detergents, the following changes in the structural and enzymatic properties of the preparation are observed: 1. The viscosity of the vesicular suspension initially rises. This change is accompanied by the formation of elongated tubules. Subsequently the membranes are completely desintegrated, resulting in a considerable reduction of the viscosity. 2. A decrease in the activity of the Ca2+-dependent ATPase, which is restored after complete solubilization. 3. A decrease in the change of intrinsic tryptophan-fluorescence on removal of calcium ions, which is also restored after complete solubilization. 4. A decrease of the calcium affinity of the ATPase. 5. A decrease in the amount of phosphorylated protein formed by the incorporation of inorganic phosphate. On the other hand, the amount of phosphoprotein formed from ATP is not affected during solubilization. 6. The dependence of the initial rates of phosphoprotein formation from inorganic phosphate on either magnesium or inorganic phosphate at low concentrations of the respective ligand changes from an S-shape profile to a normal hyperbolic profile after solubilization.