This study examined the role of muscarinic parasympathetic mechanisms in generating the progressive increases in ventilation (V˙E) and heart rate previously reported with 8 h exposures to hypoxia. The sensitivities of V˙E (Gp) and heart rate (GHR) to acute variations in hypoxia, and V˙E and heart rate during acute hyperoxia were assessed in 10 subjects before and after two 8 h exposures to isocapnic hypoxia (end-tidal PO2 = 50 mmHg). The responses were measured during muscarinic blockade with glycopyrrolate (0.015 mg kg-1) and without glycopyrrolate, as a control. There were significant increases in Gp (P < 0.01) and V˙E during hyperoxia (P < 0.01) following hypoxic exposure, but these were unaffected by glycopyrrolate. GHR increased significantly by 0.29 ± 0.08 beats min-1 %-1 (mean ± S.E.M.) following exposure to hypoxia under control conditions, but only non-significantly by 0.10 ± 0.08 beats min-1 %-1 with glycopyrrolate. This difference was significant. Changes in heart rate during hyperoxia were slight and inconclusive. We conclude that muscarinic mechanisms play little role in the progressive ventilatory changes that occur over 8 h of hypoxia, but that they do mediate much of the progressive increase in heart rate. Experimental Physiology (2001) 86.4, 529-538.