The inactivation profiles of rabbit muscle creatine phosphokinase (CPK) in heparinized whole blood and plasma under various pH conditions at 39℃ have been investigated. Under pH7.40, at 39℃, the inactivation profile of CPK in heparinized whole blood follows apparent first-order kinetics. The inactivation rate constant is O.054 h-1 and is very close to that observed in patients with acute myocardial infarction, (reported by Steele and Cohn) but much less than that resulting from exercise (reported by King et al.). The nature of the inactivation of CPK in vitro is presumably thermal denaturation, and there is no evidence to show that blood cells or dissolved oxygen are involved. The inactivation profiles of CPK in heparinized whole blood at pH other than 7.40 appear to be biphasic. The regression equations and the half-lives are presented.