The catalytic activity of rabbit skeletal muscle creatine kinase is diminished in solutions of sodium urate. The rate of inactivation follows first-order kinetics and is dependent upon urate concentrations. During the course of inactivation, only partial reactivation can be obtained by exposure to thiol agents. Reduction in the number of reactive sulfhydryl groups from eight per dimer to four accompanies inactivation of the enzyme, but no alteration in electrophoretic mobility is detectable.