The injection of contrast medium into the renal artery causes a biphasic change in renal blood flow, a transient vasodilation followed by a more prolonged vasoconstrictive phase. The role of sodium balance in the vasoconstrictive response to contrast medium was evaluated in sodium-deplete dogs (n = 5) and sodium-replete dogs (n = 5). Contrast medium, 2 ml of diatrizoate solution per 4 sec, was injected into the renal artery in anesthetized dogs. Renal blood flow was 117 +/- 25 ml/min and 173 +/- 12 in the sodium-deplete and sodium-replete dogs, respectively. After the injection of contrast medium and following the transient vasodilation phase, renal blood flow significantly decreased from baseline by 42.2% +/- 4.2 and 12.2% +/- 1.1 in sodium-deplete and sodium-replete dogs, respectively. The duration of the vasoconstriction phase was significantly prolonged in the sodium-deplete dogs, 343 +/- 27 sec vs. 147 +/- 48 in sodium-replete dogs. Blockade of the intrarenal renin-angiotensin system with the renal artery infusion of Sar1-Ala8-All (50 micrograms/kg/min) did not significantly alter the magnitude of vasoconstrictive response to contrast medium in sodium-deplete animals; however, the duration of the vasoconstrictive phase was decreased from 277 +/- 59 sec to 86 +/- 13. We conclude that (1) sodium depletion accentuates both the magnitude and duration of the vasoconstrictive phase of the renal blood flow response to injection of contrast medium and (2) blockade of the intrarenal renin-angiotensin system shortens the duration of this response.