1. We have investigated the vasoreactivity of isolated pulmonary resistance vessels of rats after acclimatization to chronic hypoxia in a normobaric, hypoxic chamber. Vasoconstriction, in response to KCl and prostaglandin F2 alpha, and vasodilation, in response to atrial natriuretic peptide, were studied isometrically in a small-vessel myograph. Resting tensions were set to simulate transmural pressures of 17.5 mmHg or 35 mmHg. 2. There were no significant differences between intergroup internal vessel diameters or maximal contractile responses to either agonist. Both control and chronically hypoxic vessels generated significantly greater active contractions at 35 mmHg than at 17.5 mmHg. There were significant positive correlations between vessel diameter and maximum contractility for both control and chronically hypoxic vessels, but when contraction was expressed as equivalent transmural pressure no correlation existed. 3. There was a significant increase in potency (as measured by the concentration necessary to produce 50% of the maximum response) of KCl in chronically hypoxic vessels compared with control vessels at 35 mmHg, but not at 17.5 mmHg. In contrast, the potency of prostaglandin F2 alpha was significantly increased in chronically hypoxic vessels at 17.5 mmHg, but not at 35 mmHg. Thus the change in contractile responses of vessels from chronically hypoxic animals, in terms of maximal response and potency, is dependent on both resting pressure and agonist used. 4. After exposure to chronic hypoxia, atrial natriuretic peptide induced significantly greater maximal relaxation of pulmonary resistance vessels at both resting pressures, but its potency was unaffected.