We have considered theoretically and experimentally a paramagnetic resonance spectrometer, whose cavity contains both the sample to be studied and a beam of ammonia molecules capable of stimulated emission. Different modes of operation are investigated: classical, ``Q‐multiplier'' and marginal oscillator spectrometer. In these two latter cases, the ultimate sensitivity is determined only by the thermal noise in the cavity. It is shown to be the theoretical sensitivity of a spectrometer using a perfect receiver. This sensitivity can be achieved if the stored energy does not exceed an upper limit, depending on properties of the active molecules‐typically 5 × 10−18J for NH3.