4-Phenyl-1H-1,5-benzodiazepin-2(3H)-one reacts in the pres-ence of a concentrated aqueous solution of sodium hydroxide and a quaternary ammonium salt (as catalyst) in benzene (phase transfer catalysis) with propargyl bromide, affording the title benzodiazepine derivative, C(18)H(14)N(2)O. In the mol-ecule, the mean plane of the propargyl substituent is almost perpendicular with that of the amide group [dihedral angle = 87.81 (8)°]. In the crystal, the molecules are linked by C-H⋯O and C-H⋯N inter-actions.