The activity within the autonomic nervous system may be altered following sustained exposure to hypoxia, and it is possible that this increase in activity underlies the early acclimatization of both ventilation and the pulmonary vasculature to hypoxia. To test this hypothesis, seven individuals were infused with the ganglionic blocker trimetaphan before and after an 8-h exposure to hypoxia. The short half life of trimetaphan should ensure that the initial infusion does not affect acclimatization to the 8-h hypoxia exposure, and the use of a ganglion blocking agent should inhibit activity within all branches of the autonomic nervous system. During the infusions of trimetaphan, measurements of ventilation and echocardiographic assessments of pulmonary vascular tone (Pmax) were made during euoxia and during a short period of isocapnic hypoxia. Subjects were also studied on two control days, when a saline infusion was substituted for trimetaphan. Trimetaphan had no effect on either euoxic ventilation or the sensitivity of ventilation to acute hypoxia. Trimetaphan significantly reduced Pmax in euoxia (p<0.05), but had no significant effect on the sensitivity of Pmax to acute hypoxia once changes in cardiac output had been controlled for. The 8-h period of hypoxia elevated euoxic ventilation (p<0.001) and Pmax (p<0.001) and increased their sensitivities to acute hypoxia (p<0.001 x2), indicating that significant acclimatization had occurred. Trimetaphan had no effect on the acclimatization response of any of these variables. We conclude that altered autonomic activity following 8 h of hypoxia does not underlie the acclimatization observed in ventilation or pulmonary vascular tone