The spectral power of low frequency oscillations of systolic arterial pressure (SAP LF) has been used as a non-invasive surrogate of muscle sympathetic nerve activity (MSNA) in both experimental and clinical situations. For SAP LF to be used in this way, a relationship must exist between SAP LF and MSNA within individuals during sympathetic activation. Using progressive central hypovolaemia to induce sympathetic activation, we hypothesised that SAP LF would correlate with MSNA in all subjects. ECG, beat-by-beat arterial pressure and MSNA were recorded in humans (n= 20) during a progressive lower body negative pressure (LBNP) protocol designed to cause presyncope in all subjects. Arterial pressure oscillations were assessed in the low frequency (LF; 0.04-0.15 Hz) domain using a Fourier transform. For the entire group, SAP LF, MSNA burst frequency, and total MSNA increased during LBNP. Values for coefficients of determination (r 2) describing the linear associations of SAP LF with MSNA burst frequency and total MSNA were 0.73 and 0.84, but rose to 0.89 and 0.98 when curvilinear fits were used, indicating that the relationship is curvilinear rather than linear. Associations between SAP LF and MSNA within each individual subject, however, varied widely for both MSNA burst frequency and total MSNA, whether derived by linear (r 2 range, 1.7 × 10 -6 to 0.99) or polynomial (r 2 range, 0.09 to 1.0) regression analysis. Similar results were obtained when relationships between low frequency oscillations in diastolic arterial pressure and MSNA were evaluated. These results do not support the use of low frequency oscillations in arterial pressure as a non-invasive measure of sympathetic outflow for individual subjects during sympathetic activation.