Mongrel dogs were anesthetised and exposed to acute hypoxia (8% or 10% O2 for 10 min) before and again either 2 or 12 days after 6-hydroxydopamine (6-OHDA, 80 mg . kg-1) pretreatment. Under normoxic conditions, systemic arterial pressure and systemic vascular resistance were reduced 2 days post 6-OHDA, but only systemic resistance was reduced 12 days post 6-OHDA. Pulmonary vascular haemodynamics were unchanged by 6-OHDA under normoxic conditions. No changes in the cardiac output, pulmonary, and systemic haemodynamic or ventilatory responses to hypoxia were observed 2 days post 6-OHDA, although the heart rate response was reduced. Similarly, 12 days post 6-OHDA, reponses to hypoxia were unchanged except for a reduced systemic pressor response. A significant transient fall in systemic arterial pressure was observed at the start of the hypoxic exposure 2 days post 6-OHDA. The ventilatory responses to hypoxia appeared to be influenced very little by the sympathetic nervous system. The results indicate that pulmonary vascular resistance is under little sympathetic influence, and that hypoxic pulmonary vasoconstriction is not mediated by the sympathetic nervous system.