In the title compound, C(14)H(13)ClN(2)O, the fused hydro-pyrimidine ring adopts an envelope conformation with one of the methyl-ene C atoms at the flap. The three-membered ring is approximately perpendicular to the attached isoquinoline ring system, with a dihedral angle of 89.44 (11)°. In the crystal, mol-ecules are linked by a weak C-H⋯π inter-action, forming a helical chain along the c axis.