Mean heart rate (HR), spectral power of HR variability in a LF and HF-band, a normalized power ratio, and baroreflex sensitivity (BRS): all these are parameters which somehow reflect sympathetic and/or vagal modulating influence on the SA-node rhythm. In this study the stationarity of these parameters in 5 minute windows is investigated during 2 hours of steady state (sitting rest condition). Mean HR appeared by far the most stable parameter. HR LF-power was the least stable parameter, showing the most variance. Even the normalized power ratio, LF/(LF+ HF), although more stable, appeared to be rather non-stationary. The hypothesis that these results could be explained by a marginally stable baroreflex BP control system, which causes a “waxing and waning” resonance power (LF-power) in HR and BP, was tested by a model of short term BP control. Baroreflex sensitivity, being a system parameter, estimated from spontaneous variations in BP and HR, showed considerable variance in the experimental data too, not reproduced by the model. The rather low coherence found between BP and HR variations makes this BRS estimation technique less reliable, and may at least partly explain this low stationarity of BRS.