Ronald G. Duggleby,* Paul V. Attwood, JohnC. Wallace, and D. Bruce Keech abstract: In the presence of avidin, progress curves for the pyruvate carboxylase (EC 6.4. 1.1) reaction show a marked decrease in the rate of reaction over a period of several minutes. These curves are quantitatively described by an equation in which the rate of catalysis by the enzyme undergoes a first-order decay from a high initial velocity to a final velocity which is close to zero. The initial velocity of the reaction is inde-pendent of avidin concentration while the decay constant in-creases linearly with increasing concentrations of avidin. These findings are in agreement with a model in which there is slow equilibration between free enzyme and the enzyme-avidin complex. Alternative models involving slow isomerization of the free enzyme or of an enzyme-avidin complex are not consistent with the data. From the slope of the decay constant vs. avidin concentration plot, therate constantfor formation of the enzyme-avidin complex was calculated to be (1.42±