Objective: The frequencies of R-R interval (RRi) and blood pressure (BP) oscillations at low- frequency (LF, 0.04-0.15 Hz) have shown clinical importance. While increased frequency of LF oscillations in BP and RRi has been observed during static handgrip exercise (HG) and muscle metaboreflex activation, a-adrenergic effects on this frequency shift are not known. We tested the hypothesis that a-adrenergic blockade inhibits the frequency shift in BP and RRi oscillations during sympathetic activation. Methods: Beat-to-beat BP and RRi were measured in 12 healthy subjects (27±3 years) at rest and during HG (20% of maximal force) and post- exercise circulatory occlusion (PECO, 5 min each) with and without a-adrenergic blockade by intravenous infusion of phentolamine (PHE). Muscle sympathetic nerve activity (MSNA) was recorded from 6 subjects. Median frequencies (Med) of BP, RRi and MSNA oscillations within LF were calculated from power spectra. Results: Without PHE, MedRRi increased during HG (from 0.073±0.009 to 0.088±0.014 Hz, p<0.01) and increased further during PECO (0.099±0.017 Hz, p<0.05 vs. HG). MedBP increased during HG (from 0.072±0.010 to 0.090±0.014 Hz, p<0.01) and sustained at higher level during PECO (0.094±0.016 Hz, p<0.01 vs. rest). PHE did not alter MedRRi and MedBP at rest and during HG but MedRRi was lower (0.082±0.019 Hz, p<0.05) and MedBP tended to be lower (0.084±0.015 Hz, p<0.1) during PECO with PHE than without. MedMSNA did not change during any intervention. (e.g. 0.102±0.009 and 0.108±0.012 Hz for rest and HG without PHE). Conclusion: a-adrenergic effects may not be responsible for the increased frequency of LF oscillations in RRi and BP during static exercise but they may contribute to the frequency shift during muscle metaboreflex activation. However, the frequency shift in LF oscillations in RRi and BP is not accompanied with altered frequency of MSNA oscillations implicating a direct a-adrenergic mechanism of LF frequency shift during PECO.