Background The hypothesis that fenoldopam mesylate, by increasing renal flow, could reduce renal damage in patients undergoing cardiac surgery with cardiopulmonary bypass has gained great interest. The aim of the current study was to quantify the relationship of the increase of the renal blood flow as a function of the fenoldopam dose in these patients and to evaluate renal flow distribution within the renal parenchyma using Doppler. Methods Twenty-five patients admitted to cardiac surgery have been enrolled. We used the Doppler technique to measure renal blood flow at the level of the renal, segmental, interlobar, and interlobular arteries. We calculated both the resistive and pulsatility indexes in all the renal segments. Moreover, we calculated echographically all the variables of preload, afterload, and cardiac index. Measurements were performed at baseline and after the infusion of fenoldopam mesylate at the doses of 0.05, 0.1, 0.2, and 0.3 μg · kg−1 · min−1. Results Fenoldopam infusion at doses more than 0.1 μg · kg−1 · min−1 significantly increases blood flow in all renal compartments, thus improving the resistive and pulsatility indexes starting at a dose of 0.1 μg · kg−1 · min−1 . The highest renal flow increase is observed with 0.3 μg · kg−1 · min−1 . Fenoldopam seems to increase the renal flow directed to the most external kidney areas. Systemic hemodynamically significant changes are observed only in patients receiving doses more than 0.1 μg · kg−1 · min−1. Conclusions In hemodynamically stable patients undergoing cardiac surgery with preserved renal function, fenoldopam shows a pharmacodynamic dose-dependent profile: it significantly increases renal flow and reduces the resistances of the renal circulation starting at a dose of 0.1 μg · kg−1 · min−1.