The radioactive-decay rate of impurity ^123mTe atoms in MgO was observed to decrease with the temperature lowered from 295 to 80 K. It is assumed that this phenomenon is connected with stimulated gamma emission of the long-lived isomer ^123mTe, whose probability should increase with decreasing temperature due to the increase of the Mössbauer factor for 88-keV gamma photons. A phenomenological description of the effect of stimulated gamma radiation (SGR) on the radioactive-decay rate is proposed, and a parameter is introduced which characterizes the SGR intensity such that, as this parameter approaches one, a single-pass laser-shot mode should set in. It is shown that in the geometry used this critical mode for MgO:^123mTe can be initiated by increasing the specific activity by two orders of magnitude.