The pH dependence of the exchange rates for a number of tryptophan and amide hydrogen atoms in hen egg-white lysozyme has been determined at temperatures well below the thermal denaturation temperature. The pH behaviour of each hydrogen is unique and can differ markedly from that of simple compounds. A model for electrostatic effects in proteins is described and used to explain a number of the features of the pH dependence of the exchange rates of certain hydrogens. The results indicate that exchange takes place from a conformation of the protein closely similar to that of the native protein, with local fluctuations providing the mechanism for exchange. For the more-buried hydrogens at low pH values there is a general increase in the exchange rates caused by the decreasing stability of the protein as calculated from the electrostatic model. The analysis shows how evidence from hydrogen exchange studies can be used to provide information about electrostatic interactions in localized regions of proteins. A description of the electrostatic model and some applications are given in the Appendix.