In order to use ferrites to modulate microwaves efficiently maximum coupling must be obtained between the microwave field and the ferrite and this can be done conveniently by using a high-Q cavity. The paper presents the analysis and measurement of the behaviour of a ferrite rod in an H11ncylindrical resonant cavity with separate input and output ports. It is shown that the cavity behaves like a pair of coupled resonant circuits, the coupling being proportional to the magnetization of the ferrite rod. The power required for using the device as a modulator is calculated in terms of theQof the cavity and the microwave attenuation, and it is shown how the modulation power may be reduced by sacrificing bandwidth and output at the microwave frequency. The problem of introducing the modulating field into the cavity is discussed. Measured results are presented for a microwave frequency of 35 Gc/s, which suggest that the device could be used as a video modulator up to frequencies of the order of 70 Mc/s with a driving power of about 25W for 100% modulation, and with a minimum microwave attenuation of 3.5 dB.