This study compared the vasoconstrictor action of angiotensin II (ANG II) and norepinephrine (NE) with different levels of nitric oxide (NO) in the kidney of anesthetized rats. In one series of experiments, the drugs were infused intravenously, and systemic NO content was reduced by a NO synthase inhibitor, nitro-L-arginine methyl ester (L-NAME). L-NAME significantly enhanced the renal blood flow (RBF) reduction produced by ANG II from 26 to 49%, but it had no significant effect on the change in RBF induced by NE. Medullary blood flow was not influenced by either ANG II or NE given alone or given after L-NAME. In the second series of experiments, all drugs were infused into the renal artery to avoid their systemic and, hence, extrarenal effects. In these experiments, renal content of NO was increased by the NO donor sodium nitroprusside (SNP), decreased by L-NAME, or restored by replacing endogenous NO by exogenous NO (L-NAME + SNP). Effects of both ANG II and NE on RBF were similarly and significantly attenuated by SNP (60% of control), enhanced by L-NAME (200% of control), and restored by L-NAME + SNP (90% of control, not significant). Our results indicate that NO attenuates the renal vasoconstriction due to ANG II or NE and that the antagonism between vasoconstrictors and NO is not due to a constrictor-induced production of NO because exogenous and endogenous NO were equally effective.