Aim: We hypothesized that changes in nitric oxide (NO) synthesis are important for the natriuretic response to saline loading, which, therefore, is blunted when NO availability is stabilized. Methods: NO-clamping was obtained by simultaneous infusions of N^G-nitro-L-arginine methyl ester (L-NAME; 750 mg•kg^-1 bolus and 8.3 mg•kg^-1•min^-1) and sodium nitroprusside, the latter at rates necessary to prevent the L-NAME to increase total peripheral resistance. The hypothesis was tested by slow volume expansion (SVExp, saline, 20 mmol NaCl•kg^-1•min^-1 for 3 h) with and without NO-clamping. Results: NO-clamping was associated with small decreases in the baseline values of renal plasma flow. SVExp decreased renin system activity: plasma renin, AngII, and aldosterone all fell significantly, while heart rate, arterial blood pressure (ABP, oscillometry), cardiac output (impedance cardiography), renal plasma flow (PAH), glomerular filtration rate (GFR by ⁵¹Cr-EDTA), plasma [Na⁺ and [K⁺] remained constant. SVExp increased sodium excretion at constant filtered load of sodium; however, more so during NO- clamping than during control SVExp. Plasma nitrate/nitrite levels (NOx) tended to decrease irrespective of clamping; plasma cGMP and plasma vasopressin remained constant. Conclusion: The results disprove the working hypothesis. NO-clamping augments sodium excretion in response to volume expansion at constant ABP and GFR. Changes in renin system activity were very similar. The results indicate that mediator (s) other than ABP, overall renal haemodynamics, and renin system activity are important in the regulatory response to sodium/volume loading.