Fluorescence polarization and formation of excimers were studied in N-(3-pyrene)maleinimide-labeled sarcoplasmic reticulum vesicles. 1. The polarization of pyrenemaleinimide labeled vesicles does not change with temperature and shows a pronounced decrease at labeling concentrations larger than 1 mol pyrenemaleinimide per 10 mol ATPase. 2. Solubilization of the membrane with myristoylglycerophosphocholine renders the polarization temperature dependent, but does not affect the concentration dependent depolarization observed in native vesicles. 3. The polarization of labeled vesicles is much smaller than to be expected from the temperature independent polarization indicating that the pyrenemaleinimide polarization did not monitor the rotation of the entire ATPase. Thus segmental motion occurs. 4. Pyrene excimers are observed at label concentrations larger than 1 mol label per 2.5 mol ATPase. 5. The amount of excimers was critically dependent on added detergents. From the fact that non-solubilizing amounts of myristoylglycerophosphocholine strongly reduced the amount of pyrene excimers it is concluded that in the native sarcoplasmic reticulum vesicles at least two ATPase molecules must be in close contact.