Abstract It is well established that after acclimatization at high altitude, many sympathetic pathways are hyperactive yet heart rate (HR) remains unchanged. In this study, we attempted to determine if this unchanged heart rate is due to compensatory mechanisms such as changes in parasympathetic activity or levels of receptors for autonomic neurotransmitters. We also examined the role played by hypoxia in these autonomic adaptations to high altitude. Three experiments were carried out on five healthy lowlanders both at sea level (SL) and after 2 weeks of acclimatization at 3800 m (Post-Ac) with: (a) placebo (control); (b) acute β-adrenergic receptor blockade by propranolol (PRO), or (c) acute parasympathetic receptor blockade by glycopyrrolate (GLY). Compared with SL control values, post-Ac venous norepinephrine (NE) and dopamine increased by 96% ( p<0.001) and 55% ( p<0.05), but epinephrine and HR did not change. PRO resulted in a smaller decrease in HR (bpm) Post-Ac than at SL (15±6 vs. 21±6, p<0.05), while GLY caused a greater increase in HR Post-Ac than at SL (59±8 vs. 45±6, p<0.05). Breathing oxygen at SL concentration while at altitude did not decrease NE, or alter the effect of PRO on HR, but reduced the chronotropic effect of GLY by 14% ( p<0.05). These results suggest that after acclimatization to altitude, increased parasympathetic neurotransmitter release and decreased β-adenoreceptor activity account for the unchanged HR despite enhanced sympathetic activity. Acute oxygen replacement rapidly counteracted the parasympathetic, but not sympathetic hyperactivity that occurs at high altitude.