The transient growth of the spin‐wave population in a microwave magnetic field applied parallel to the dc field is investigated. It is assumed that the sample is in thermal equilibrium before the initiation of the microwave pulse, and that the rf field amplitude remains constant during the pulse. According to the theory the absorption coefficient approaches a steady‐state value below threshold, and grows indefinitely (as long as the field amplitude remains constant) above the threshold. The results of detailed calculations concerning the steady‐state and the transient absorption are described. Above threshold the time dependence of the absorption coefficient is characterized by a very fast initial rise to an intermediate plateau or hump, a subsequent more gradual rise, and an approximately exponential rise at later times. As the absorption coefficient increases, the field amplitude in the cavity decreases (assuming that the incident power remains constant). This reaction of the sample upon the field amplitude is taken into account in an approximate fashion. Because of this reaction the absorption coefficient approaches a new steady state above the threshold.