The potential energy hypersurface of metisazone, which is an antiviral preparation of a wide spectrum antiviral action has been first analyzed by semi-empirical quantum chemical method AM1. The hypersurface was shown to have one global and seven local minima corresponding to four planar conformations (one of which being a main conformation) and two pairs of substantially non-planar mirror-symmetrical conformations. All these conformations fall within the Q+14.7 kcal/mole range of relative energy and are highly polar structures. The highest dipole moments correspond to the basic conformation (6.8 D) and the most unstable conformation (8.5 D). It has been first established by semi-empirical quantum chemical method MNDO/H that the main conformation is stabilized by at least two intramolecular H-bonds NH...O=C and NH...N, each of them with the energy of 2.5 kcal/mole. Vibrational spectrum of metisazone contains four low fundamental frequencies (39, 46, 69 and 81 cm^-1 ), which evidences its structural nonrigidity.