- It is shown that the celebrated Rigrod model [J. Appl. Phys. 34, 2602 (1963)] is inadequate for calculating the output power of gas-flow lasers in an important range of parameters in which the quenching of the excited species is slow and the optical extraction efficiency is high. For two-level systems very simple analytical formulas for the output power are obtained that should be used instead of the Rigrod model. The formulas are given for Fabry–Perot, stable, and unstable resonators. Both the saturation parameter and the extraction efficiency differ from those appearing in the Rigrod model. Applications of the formulas to systems with more complex kinetic schemes are also indicated.