We have reported that the renal hemodynamic effects of norepinephrine (NE) are modulated by cyclooxygenase-2 (COX-2)-derived metabolites. Our main objective was to examine whether there is an interaction between nitric oxide (NO) and COX-2 in modulating the renal hemodynamic effects of NE. NE was infused at 3 doses to anesthetized dogs pretreated with: vehicle (n=8); a selective COX-2 inhibitor (nimesulide) (n=6); a NO synthesis inhibitor (L-NAME) (n=8); or with nimesulide and L-NAME (n=5). During NE infusion, PGE₂ excretion increased (125%) in the control group and did not change in the L-NAME-treated dogs. The simultaneous inhibition of NO and COX-2 potentiated to a greater extent the NE-induced renal vasoconstriction than inhibition of either NO or COX-2. The NE-induced renal vasoconstriction during NO and COX-2 inhibition was reduced (P<0.05) by infusing an AT₁ receptor antagonist (n=6). These results suggest that there is an interaction between NO and COX-2 in protecting the renal vasculature from the NE effects and that angiotensin II partly mediates the NE-induced renal vasoconstriction when NO synthesis and COX-2 activity are reduced.