- The decay characteristics of two-state Rydberg atoms are investigated in a microwave cavity driven simultaneously by two strong rf fields. Tuning one of the driving fields leads to sharp resonances in the decay rate of the upper atomic level. Their width may become smaller than the cavity linewidth and transit time broadening. We present a theoretical model, interpreting the resonances as cavity-induced decay of Floquet states of the composite atom-field system. The observed phenomena arise from the interplay of cavity-modified Bloch-Siegert shifts and dynamic enhancement of spontaneous emission at single-photon and multiphoton resonances. 1994 The American Physical Society.