The object of this study is to investigate the effect of VIP on pulmonary artery of chronically hypoxic rats. It was shown that chronic hypoxia depressed significantly pulmonary artery relaxation induced by VIP as compared with those of control （P<0. 001）. The vascular relaxation of both groups was correlated with concentration of VIP. In addition

the relaxant effect of VIP on pulmonary arteries in rats was endothelium─independent

and was not prevented by indomethacin or nordihydroguaiaretic acid

but was abolished completely by methylene blue. These results suggest that the lower relaxation of pulmonary artery in rats might not be due to the endothelial injury caused by chronic hypoxia

and chronic hypoxia may inhibit directly the soluble guanylate cyclase in vascular smooth muscle cells invloved in synthesis of cGMP and thus reduced the sensitivity and reactivity of pulmonary artery to VIP.