Peptide hydrogen exchange is measured in bovine pancreatic trypsin inhibitor (BPTI) by 2D NMR in KCl solutions varying between 0.02 and 0.43 M. The effects of salt are analyzed for 16 assigned peptide groups located near the protein-solvent interface in the crystal structure. Salt effects are obtained for exchange by H+ and OH- catalysis, at pH 2.3 and 5.3, respectively. Semilogarithmic plots of rate constants vs the square root of the ionic strengths are virtually linear. The salt effects, taken as the slopes of these plots, vary both in size and sign for each catalytic process, reflecting the variation of local electrostatic field at the exchanging site. The effects are correlated with electrostatic potentials calculated by the finite differences method, taking into account both ionic and dipolar charges in the static structure. This suggests that the transition complexes between the catalyst and the protein are formed with the protein structure very similar to the crystal structure.