The heats of interaction between glycyl-glycyl-glycine and solutions of nitric acid and potassium hydroxide are determined by calorimetry at 298.15 K and ionic strengths of 0.25, 0.50, and 0.75 in the presence of KNO₃. The standard thermodynamic characteristics (Δ_rH°, Δ_rG°, and Δ_rS°) of acid–base interaction in aqueous solutions of the peptide are calculated. The effect of background electrolyte concentration on the enthalpy of dissociation of glycyl-glycyl-glycine is discussed.