The paper reports a study on the kinetics of the transition between acid and alkaline Aplysia myoglobin. The results, obtained with the temperature-jump method, show that the observed relaxation effect corresponds to a perturbation of the equilibrium between the protonated and deprotonated forms of the protein. The pH dependence of the data and their analysis suggest that the process involves a proton-linked conformational change of the protein, in agreement with independent experiments. The comparison of the results reported here with those obtained for other hemeproteins, notably sperm whale myoglobin, constitutes a clear-cut example of the dominant role of the protein moiety in regulating the ligand reactivity of the heme.