Changes in fluorescence intensity of a membrane-embedded probe were used to study the kinetics of binding of oxy- and deoxyhemoglobin to erythrocyte membranes. For these studies, stopped-flow fluorimetric techniques were utilized. Both binding and dissociation of hemoglobin from membranes followed heterogeneous first-order kinetics. The rate constants for binding of oxyhemoglobin were about 10 times larger than those of deoxyhemoglobin; the dissociation rate constants of oxyhemoglobin were about one-quarter those of the unliganded form. The results are discussed in light of the steady-state binding constants previously derived for both oxy- and deoxyhemoglobin.