Dissolution enthalpies of (L)-alpha-proline, (L)-alpha-tyrosine, (L)-alpha-tryptophan, (L)-alpha-histidyne, (L)-alpha-arginine, (L)-alpha-lysine, (L)-aspartic acid, and (L)-alpha-glutarnic acid in aqueous solutions of urea have been measured by calorimetry at a temperature of 298.15 K. The values of dissolution enthalpy were used to determine enthalpic heterogeneous pair interaction coefficients between the zwitterions of the natural amino acids and a molecule of urea in water solution. These coefficients were interpreted in terms of the hydrophobic or hydrophilic effects of the side chains of amino acids on their interactions with a polar molecule of urea in water.