To study spontaneous cardiac baroreflex at different timescales, a new method has been developed that identifies such episodes. Mean arterial pressure (MAP) and heart rate (HR) were recorded beat to beat over 1 h in freely moving control (n = 10) and acutely (1 day before study, n = 7) and chronically (2 wk before study, n = 10) sinoaortic-denervated (SAD) 12- to 14-wk-old male Sprague-Dawley rats. These beat-to-beat time series were successively low-pass filtered seven times and resampled at different time intervals from 0.1 to 6.4 s, allowing different timescales to be scanned. With the use of the Z coefficient, the statistical relationship was estimated for the associations of inverse MAP and HR variations when these inverse MAP and HR variations occurred simultaneously or were time shifted. In control rats and for timescales > or = 0.4 s, the highest Z coefficient (0.38) was obtained when MAP variations preceded inverse HR variations by one sampling interval. The baroreflex origin of this link was demonstrated by its disappearance after acute SAD. In conclusion, this method enabled spontaneous baroreflex episodes to be identified for unusually long timescales without limiting the study to fast, linear, stationary, or oscillating phenomena.