The overall thermal denaturation of glycogen phosphorylase b is irreversible and our results conform to the theoretical prediction of a reversible process followed by a slower irreversible process. The basic thermodynamic parameters of glycogen phosphorylase b denaturation have been worked out and found to be: critical temperature 57.0 +/- 0.5 degrees C, transition half-width 8 +/- 1 degrees C, and calorimetric enthalpy change and Van't Hoff enthalpy change of the denaturation process 450 +/- 50 and 105 +/- 15 kcal/mol of enzyme monomer, respectively, at pH 7.4. These parameters have been found to be largely altered by the detergents octylglucoside, cholate, and deoxycholate at or below their critical micelle concentration, but not by Triton X-100 nor by lecithin liposomes. Organic solvents, such as dimethyl sulfoxide and methanol, and the presence of sarcoplasmic reticulum membranes produces an alteration of the denaturation thermogram of glycogen phosphorylase b similar to that produced by the above-mentioned detergents. These results allow us to hypothesize that hydrophobic domains of glycogen phosphorylase b are involved in its association to sarcoplasmic reticulum membranes in the sarcoplasmic reticulum/glycogenolytic complex of mammalian skeletal muscle.