In this work, the gas-phase homolytic N-H bond dissociation enthalpy (BDE) was investigated for a large series of molecules containing at least one N-H bond by means of accurate density-functional theory calculations. The molecules studied belong to different classes of compounds, namely, amines, amides and anilines, amino acids, phenoxazines, indolamines, and other compounds of general interest, such as anti-inflammatory drugs. To achieve these purposes, the (RO)B3LYP/6-311+G(2d,2p)//(U)B3LYP/6-31G* level of theory was used. The calculated gas-phase N-H BDEs, at T = 298.15 K, are in the range 499.6-203.9 kJ/mol, for purine and HNO, respectively. Further, the calculated BDEs are in excellent agreement with a significant number of available experimental BDEs. Solvent effects were also taken in account, and rather significant differences are found among N-H BDEs computed in the gas phase and in heptane, DMSO, or water.