Structural transitions induced by temperature in the lipid bilayer of rabbit sarcoplasmic reticulum membranes have been detected by means of the hydrophobic fluorescent probe pyren. Using the phenomenon of energy migration from membrane protein tryptophan residues to pyren a correct estimate of lipid viscosity near protein molecules was made by an empirical formula. The structural transitions of free and protein-bound lipids proceeded at the same temperature intervals. It was found that viscosity of protein-bound lipids was higher than that of free lipids, but they became equal at 36--39 degrees C.