Arterial pressure fluctuates rhythmically in healthy supine resting humans, who, from all outward appearances are in a 'steady-state'. Others have asked, If baroreflex mechanisms are functioning normally, how can arterial pressure be so variable? We reanalysed data from nine healthy young adult men and women and tested the hypotheses that during brief periods of observation, human baroreflex sensitivity fluctuates widely and rhythmically. We estimated vagal baroreflex sensitivity with systolic pressure and R-R interval cross-spectra measured over 15 s segments, moved by 2 s steps through 20 min periods of frequency- and tidal volume-controlled breathing. We studied each subject at the same time on three separate days, with fixed protocols that included two physiological states, supine and passive 40° upright tilt, before and after -adrenergic, cholinergic, and angiotensin converting enzyme blockade. Minimum, mean, and maximum (±S.D.) supine control baroreflex sensitivities averaged 5 ± 3, 18 ± 6, and 55 ± 22 ms/mmHg. In most subjects, moderate ongoing fluctuations of baroreflex sensitivity were punctuated by brief major peaks, yielding frequency distributions that were skewed positively. Fast Fourier transforms indicated that baroreflex sensitivity fluctuations (expressed as percentages of total power) concentrated more in very low, 0.003 - 04 Hz, than ultra low, 0.0 - 0.003 Hz, frequencies (77 ± 7 vs 11 ± 8 %, P 0.001, rank sum test). Autoregressive centre frequencies averaged 0.012 ± 0.003 Hz. The periodicity of very low frequency baroreflex sensitivity fluctuations was not influenced significantly by upright tilt, or by variations of autonomic drive or angiotensin activity. Our analysis indicates that during ostensibly 'steady-state' conditions, human vagal baroreflex ensitivity fluctuates in a major way, at very low requencies.