1. Sarcoplasmic reticulum membranes were labelled with 1-dimethylaminonaphtalene-5-sulfonyl chloride (DnsCl). Analyses of the dansylated membranes demonstrated that the most of the dye was associated with ATPase (ATP phosphohydrolase, EC 3.6.1.3) and phosphatidylethanolamine in the membranes. 2. Dansylation of the membranes could be performed without significant decrease in the ATPase activity. 3. Partial differentiation of fluorescence of Dns-phosphatidylethanolamine from that of Dns-ATPase could be achieved by changing excitation wavelength; Dns-ATPase emmitted in the shorter wavelength region, while Dns-phosphatidylethanolamine emmitted in the longer wavelength region. 4. Fluorescence polarization of the dye bound to the membranes indicated that both the ATPase and phosphatidylethanolamine were strongly immobilized in the membranes, while the ratio of freely rotating dye to the "frozen" dye bound to the ATPase was larger than that bound to the phosphatide.