Our prior studies indicated that postural fainting relates to thoracic hypovolemia. An supranormal increase in initial vascular resistance was sustained by increased peripheral resistance until late during head-up tilt (HUT), while splanchnic resistance, cardiac output and BP decreased throughout HUT. Our aim in the current study was to investigate the alterations of baroreflex activity that occur in synchrony with the beat-to-beat time dependent changes in HR, BP and TPR. We proposed that changes of low frequency Mayer waves reflect sympathetic baroreflex. We used discrete wavelet transform multiresolution analyses to measure their time dependence. We studied 22 patients 13 to 21 years old, 14 who fainted within 10 minutes of upright tilt (fainters), and 8 healthy control subjects. Multiresolution analysis was obtained of continuous blood pressure, heart rate and respirations as a function of time during 70° upright tilt at different scales corresponding to frequency bands. Wavelet power was concentrated in scales corresponding to 0.125 Hz and 0.25 Hz. A major difference from control subjects was observed in fainters at the 0.125 Hz arterial pressure scale which progressively decreased from early HUT. The alpha index at 0.125 Hz was increased fainters. RR-interval 0.25 Hz power decreased in fainters and controls but was markedly increased in fainters with syncope and thereafter corresponding to increased vagal tone compared to control subjects at those times only. The data imply a rapid reduction in time-dependent sympathetic baroreflex activity in fainters but not control subjects during HUT.