We have developed a new methodology for calculating contributions to the rate constants ( k IC ) of internal conversion that are induced by external electric ( k IC-E ) or magnetic ( k IC-M ) fields. The influence of the external electric and magnetic fields on the k IC was estimated for seven representative molecules. We show that the k IC-E contribution calculated at a field strength of 10 11 V m −1 is generally as large as the k IC rate constant in the absence of the external field. For indocyanine green, azaoxa8circulene, and pyromitene 567, the k IC-E contribution is as large as k IC already at a field strength of 10 9 V m −1 . Such electric-field strengths occur for example in plasmonic studies and in strong laser-field experiments. The induced effect on the k IC rate constant should be accounted for in calculations of photophysical properties of molecules involved in such experiments. The induced effect of an external magnetic field on k IC can be neglected in experiments on Earth because the magnetic contribution becomes significant only at very strong magnetic fields of 10 4 -10 5 T that cannot be achieved on Earth. However, the magnetic effect on the rate constant of internal conversion can be important in astrophysical studies, where extremely strong magnetic fields occur near neutron stars and white dwarfs. We have developed a new methodology for calculating contributions to the rate constants ( k IC ) of internal conversion that are induced by external electric ( k IC-E ) or magnetic ( k IC-M ) fields.