Background/Aims: Contrast media (CM) induce a biphasic renal hemodynamic response, with late prominent cortical vasoconstriction and marked outer medullary vasodilation. The objective of the study was to explore a possible role for altered nitric oxide (NO) production or bioavailability in these hemodynamic responses. Methods: We explored the impact of CM (sodium iothalamate) upon rat renal NO synthase (NOS) activity (citrulline recovery) and NO (using a NO electrode). Results: The cortical NOS activity following CM was 11.5 ± 1.0 versus 13.8 ± 1.1 nmol/gww/min (gww = gram wet weight) in controls (p = 0.16, NS). In rats pretreated with the nonselective endothelin antagonist bosentan, CM reduced the cortical NOS activity to 8.5 ± 1.2 nmol/gww/min (p < 0.005 vs. controls). Cortial NO readings declined over 30 min following CM by 13 ± 8% (p < 0.05, Anova), in parallel with the decline in cortical blood flow. The outer medullary NOS activity was not affected by CM (5.2 ± 1.5 vs. 5.5 ± 1.3) nmol/gww/min in controls) or bosentan. Nevertheless, the outer medullary NO reading increased by 36 ± 23% (p < 0.05), with a concomitant increase in regional blood flow. Conclusion: In the cortex, CM might reduce the NOS activity (an effect blunted by endothelin release). This may potentiate the effect of endothelin to induce regional vasoconstriction. In the outer medulla, the vasodilatory response to CM does not seem to be mediated by enhanced NOS activity and might reflect increased local NO bioavailability as the result of regional hypoxia.