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We compared the instantaneous dynamics of spectral measures of cardiovascular variability, estimated by a time-frequency distribution (TFD), during two opposite respiratory strains, Mueller (MM) and Valsalva maneuvers (VM), and evaluated the relations among systolic pressure (SP), RR intervals (RRi) and spectral indexes. From the power spectra of RRi and SP series, estimated by TFD, the high frequency power (HFRR), low to high frequency ratio and low frequency power of SP (LFSP) were computed. During both maneuvers the instantaneous dynamics of their spectral indexes were similar, being the changes 50% smaller in MM (p<;0.001). In phase II of both maneuvers the SP-HFRR, RRi-HFRR and LFSP-SP relations showed strong correlations that support the baroreflex participation in the two maneuvers, provide a better estimate of the vagal baroreflex sensitivity, and establish the contribution of vasomotor sympathetic outflow to the SP overshoot.