The title compound, C(19)H(17)ClN(4), was obtained from the reaction of 3-chloro-5,6-diphenyl-1,2,4-triazine with isobutyronitrile in the presence of lithium diisopropyl-amide as an unexpected product of covalent addition of isobutyronitrile carbanion to the C-5 atom of the 1,2,4-triazine ring. The 2,5-dihydro-1,2,4-triazine ring is essentially planar (r.m.s. deviation = 0.0059 Å) and the 5- and 6-phenyl substituents are inclined to its mean plane with dihedral angles of 89.97 (4) and 55.52 (5)°, respectively. Intra-molecular C-H⋯N inter-actions occur. In the crystal, mol-ecules related by a c-glide plane are linked into zigzag chains along [001] by N-H⋯N hydrogen bonds.