Résumé : Objective: The aim was to characterise stimulus-response curves for hypoxic pulmonary vasoconstriction and to observe the effects of drugs reputed to enhance it: aspirin (a cyclo-oxygenase inhibitor), and doxapram (a peripheral chemoreceptor agonist). Methods: Mean pulmonary artery pressure (Ppa) versus fraction of inspired O2 (FIO2) relationships were studied in 18 intact anaesthetised piglets, before and after the intravenous administration, in random order, of either physiological saline, 1 g aspirin, or 20 mg·kg-1 doxapram. Cardiac output (Q) was kept constant, to avoid passive Q dependent changes in Ppa. Results: A progressive decrease in FIO2 from 100% to 12% was associated with an average increase in Ppa from 19 to 38 mm Hg (p<0.001). When FIO2 was further decreased to 8%, Ppa decreased to 32 mm Hg (p<0.01). This stimulus-response curve was unaffected by saline, but displaced in a non-PO2-dependent manner to higher Ppa by doxapram and by aspirin. Conclusions: The pulmonary vascular response to inspiratory hypoxia in intact anaesthetised piglets is biphasic, with a maximum at an FIO2 of 12%. Neither aspirin nor doxapram affect the shape of this stimulus-response curve, and in particular do not prevent low FIO2 associated inhibition of hypoxic pulmonary vasoconstriction.Cardiovascular Research 1992;26:944-949.