The exchange reaction of the peptide NH protons of a microbial protease inhibitor ( Streptomyces subtilisin inhibitor) with deuterium atoms in 2H 2O (p 2H 6.8) has been studied by proton magnetic resonance in the temperature range 56–71°C. Both slowly and rapidly exchanging processes have been observed. The number of slowly exchanging protons is estimated to be 25 ± 2 per subunit of the protein molecule. The decay of the slowly exchanging proton signals follows a single time-exponential function at each temperature. The observed first-order rate constants have been analyzed to give the denaturated fraction of the protein as a function of temperature with a consequent enthalpy (56 kcal/mol) and an entropy (137 cal/degree per mol) of denaturation. The results indicate the high conformational stability of this protein against heat denaturation.