1. Modelling studies have led to the proposal that Mayer waves ( approximately 0.4 Hz in rats) could result from a resonance phenomenon in a feedback control loop. In this study, we investigated the presence of a resonance frequency in the arterial baroreceptor reflex loop, i.e. a particular frequency at which arterial pressure feeds back positively to the baroreceptors. 2. Frequency responses of mean arterial pressure (MAP) to aortic depressor nerve (ADN) stimulation were studied in fifteen urethane anaesthetized, ventilated rats with cardiac autonomic blockade. The ADN was stimulated using rectangular trains of impulses (2 ms, 100 Hz) delivered at frequencies ranging from 0.1 to 1 Hz. Phase angles between impulses and MAP were calculated using cross-spectral analysis based on a fast Fourier transform algorithm. 3. Rhythmic ADN stimulation induced regular MAP oscillations at the expected frequencies that were attenuated by alpha-adrenoceptor blockade and abolished after ganglionic blockade. The relationship between impulse and MAP oscillations was characterized by a strong coherence and a positive phase shift at low frequencies, indicating that impulses led MAP with respect to the out-of-phase pattern. Deviation of the phase from the out-of-phase behaviour was mainly due to the presence of a fixed time delay ( approximately 0.8 s) between ADN stimuli and MAP changes. Phase angles fell to zero at 0.42 +/- 0.02 Hz. 4. In rats, the arterial baroreceptor reflex exhibits a resonance frequency close to the frequency of spontaneously occurring Mayer waves. The reflex therefore seems the most likely origin for the Mayer waves.