The kinetics of spontaneous inactivation of the digitonin-solubilized rat brain muscarinic cholinoreceptor was investigated at 4 degrees, 15 degrees, 25 degrees, 35 degrees and 45 degrees C. The inactivation process was followed by the loss of specific L-[3H] quinuclidinyl benzilate binding capacity after incubation of the receptor at an appropriate temperature. Since the inactivation process of the receptor inactivation obeys the first order kinetics, it was possible to determine the values of inactivation rate constants (kappa in). It was shown that the inactivation rate does not depend on the detergent excess in a reaction mixture and is characterized by the apparent activation energy, Ea = 158 +/- 7 . KJ/mole and entropy, delta S not equal to = 249.8 J/K . mole. These values are in good agreement with those obtained for the water-soluble proteins, but differ essentially from the analogous values for the spontaneous activation of the membrane-bound receptor.