We investigated the interaction between nitric oxide and the renin angiotensin system in regulating isolated pulmonary arterial tension and pulmonary arterial pressure (PAP) in renal hypertensive rats (RHR) made by complete ligation of left renal artery. Losartan induced a depressor response that was smaller in RHR than in normotensive rats (NR) (3.3 and 7.0 mmHg, respectively, at 3.0 mg/kg, p<0.05), and the response was significantly reduced by $N^{G}$-nitro-Larginine methyl ester (L-NAME). Angiotensin II elevated the PAP (7.6 and 10.8 mmHg at $0.1 {mu}g/kg$; 20.3 and 23.6 mmHg at $1.0{mu}g/kg$, respectively) and contracted the isolated pulmonary artery ($pD_2$: 8.79 and 8.71, respectively) from both NR and RHR with similar magnitude, and these effects were significantly enhanced by L-NAME in NR, but not in RHR. Acetylcholine lowered the PAP slightly less effectively in RHR than in NR (3.8 and 6.0 mmHg at 10 .mu.g/kg, respectively) and relaxed the pulmonary artery precontracted with norepinephrine in both rats with similar magnitude ($E_max$: 60.8 and 63.6%, respectively), and the effect being completely abolished after pretreatment.with L-NAME or removal of endothelial cells. These results suggest that nitric oxide interacts with renin angiotensin system to control the pulmonary vascular tension and pulmonary arterial circulation of RHR.R.