We report on electric deflection experiments of aminobenzonitrile and ( dimethylamino) benzonitrile molecules. They are used as prototypes to study the influence of the asymmetry and rotation-vibration couplings in deflection experiments. Experimental deflection profiles are compared to results of ab initio calculations in the frame of the rigid rotor Stark effect and of the statistical linear response. The change in symmetry and the introduction of methyl groups lead to a transition from the rigid rotor response to the linear response. From the experimental results, a total dipole of mu = 6.2 +/- 0.6 D has been deduced for the m-(dimethylamino)-benzonitrile molecule (MDMABN).