The role of nitric oxide (NO) and prostaglandin (PG) in modifying renal hemodynamics was examined in clipped and non-clipped kidneys of unilateral renal artery stenosis. Chronic unilateral renal ischemia was established by 4-week-clipping the left renal artery of canine kidneys, and renal interstitial nitrate+nitrite and PGE₂ contents were evaluated by the microdialysis technique. Unilateral renal artery stenosis caused 45±1% and 73±1% decrements in RPF in moderate- and severe-clipped kidneys, and 21±3% decrements in non-clipped kidneys with severe stenosis. Renal nitrate+nitrite decreased in moderate- (-31±1%) and severe-clipped kidneys (-63±4%). N-nitro-L-arginine methylester reduced RPF (-56±3%) and GFR (-54±3%) in moderate-clipped kidneys, whereas this inhibitory effect was abolished in severe-clipped kidneys. In contrast, renal PGE₂ contents increased modestly in moderate clipping, and were markedly elevated in severe-clipped kidneys (from 111±7 to 377±22pg/ml); sulpyrine impaired renal hemodynamics only in severe-clipped kidneys. In contralateral non-clipped kidneys, although renal PGE₂ was not increased, sulpyrine reduced RPF (-32±1%) and GFR (-33±3%) in severe stenosis. Collectively, NO plays a substantial role in maintaining renal hemodynamics both under basal condition and in moderate renal artery stenosis, whereas the contributory role shifts from NO to PG as renal artery stenosis progresses. Furthermore, since intrarenal angiotensin II is reported to increase in non-clipped kidneys, unilateral severe ischemia may render the non-clipped kidney susceptible to PG inhibition.