We look at the source equation for the electric vector of radiation inside a maser cavity (W. E. Lamb jr.) and investigate if a volume density of polarization P = ( 2 -Φ/c2)E, induced by gravitation (potential Φ) in space with an electric vector E, can stimulate maser oscillations (vacuum of course is our acting medium). We consider the case of single-mode excitation. We see that, if Φ does not vary with time, we get frequency shift only. Wave amplification would necessarily require a time-dependent Φ. Assuming a periodicity Tn = 2π/vn in Φ, where vn is the angular frequency of the excited mode of radiation in the cavity resonator, we see that the 2 vn Fourier component in Φ is responsible for maser amplification. For the shift of frequency we observe that it is of the same order as the gravitational red-shift. Examples are constructed with some optical resonators. © 1973 Società Italiana di Fisica.