The possibility of obtaining stimulated emissions in semiconductors has been considered for transitions between the conduction band and the valence band, or between one band and an impurity level. If the occupation of the bands and of the impurity levels is taken into account by quasi-Fermi levels, the necessary condition for stimulated emission to be possible turns out to be simply: ΔF > hv where ΔF is the difference of the quasi-Fermi levels of the initial and final state, and v is the emitted frequency. The existence of such quasi-Fermi levels is discussed, and it is shown that the above condition is due to the second law of thermodynamics. Direct interband transitions in InAs or InSb, and transitions between the conduction band and Zn and In acceptor levels, respectively, in Ge and Si are thought to be sufficiently attractive to be studied experimentally.