The protomeric tautomerizm and conformation of the 2-methyl-4-pyridin-2'-yl-1,5-benzodiazepine molecule were investigated, and its three neutral tautomers (B-1, B-2, B-3) and their rotamers (C-1, C-2, C-3) were considered. Full geometry optimizations were carried out at the HF/6-31G* and B3LYP/6-31G* levels in gas phase and in water. The tautomerization processes in water (epsilon = 78.54) were studied by using self-consistent reaction field theory. The calculation showed that the boat conformation is dominant for the seven-membered diazepine ring in all of the structures, even with different double bond positions. The calculated relative free energies (Delta G) showed that the tautomer C-1 was the most stable structure, and its conformer B-1 was the second most stable in the gas phase and in water.