We present some considerations for the inelastic increase in conductivity in scanning tunneling microscopy (STM) by using the dipole approximation for the vibrating molecules. We show that the relative increase in the case of vacuum tunneling can be considerably larger than that obtained in tunneling through oxide layers, even for a single adsorbed molecule. With the present state of STM, such current changes are readily detectable. We further propose an alternative experimental approach unique for STM. © 1985 The American Physical Society.