The kinetic theory of the substrate reaction during modification of enzyme activity has been applied to a study of the dephosphorylation of phosphorylase a by protein phosphatase-1 (ppase-1). On the basis of the kinetic equation of the substrate reaction in the presence of ppase-1, all the inactivation rate constants for the free enzyme and the enzyme-substrate(s) complexes have been determined. Binding of the allosteric substrate, glucose 1-phosphate, to one subunit of phosphorylase a protects completely against ppase-1 action on either the same subunit or the adjacent subunit, whereas binding of the non-allosteric substrate, glycogen, to one subunit protects this subunit partially, but has no effect on the modification on the neighbouring subunit. Analysis of the data suggests that the allosteric behaviour of phosphorylase a can be interpreted in terms of a modified concerted model. The present method also provides a novel approach for studying dephosphorylation reactions. Since the experimental conditions used resemble more closely the in vivo situation where the substrate is constantly being turned over while the enzyme is being modified, this new method would be particularly useful when the regulatory mechanism of the reversible phosphorylation reaction toward certain enzymes is being assessed.