1. Relaxant responses to six vasodilator drugs, with different mechanisms of action, were examined on noradrenaline (0.1 microM)-contracted ring preparations of pulmonary artery and aorta taken from rats with pulmonary hypertension induced by monocrotaline or chronic hypoxia. 2. On pulmonary artery preparations from monocrotaline-treated rats, compared with controls, (a) the maximum relaxation to pinacidil and cromakalim was significantly increased, but their potency (negative log EC50) was unchanged, (b) the potencies of nitroprusside and sodium nitrite were significantly reduced (10 fold and 3 fold respectively), but there was no change in the maxima, (c) for nicorandil there was an increase in maximum relaxation and a decrease in potency (3 fold), and (d) for atriopeptin II there was no change in potency or maximum. 3. The increase in maximum relaxation for pinacidil and the decrease in potency for nitroprusside were also demonstrated in pulmonary artery preparations from rats with chronic hypoxic pulmonary hypertension. The other four drugs were not examined in preparations from hypoxic rats. 4. In both models of pulmonary hypertension, no change in maximum response or potency was seen on aortic preparations for any of the vasodilator drugs. 5. In control preparations, none of the drugs was more potent on pulmonary artery than on aorta (i.e. they were not pulmonary-selective). In preparations from pulmonary hypertensive rats, pinacidil was selective for pulmonary artery, in contrast to nitroprusside which was selective for aorta.6. It is concluded that the development of pulmonary hypertension in rats is accompanied by changes in the responsiveness of the pulmonary arteries to some vasodilator drugs; whether or not these changes occur depends on the mechanism of action of the vasodilator drug, but they are independent of the method of inducing pulmonary hypertension.7. It is postulated that the reduction in potency seen for nitroprusside, sodium nitrite and nicorandil may be due to desensitization of soluble guanylate cyclase in pulmonary vascular smooth muscle in pulmonary hypertension.