The purpose of this study was to determine the role of the renal nerves in the natriuresis and diuresis that is observed with the systemic infusion of a high dose of NG-monomethyl-L-arginine (L-NMMA) to inhibit nitric oxide synthesis in the Okamoto spontaneously hypertensive rat (SHR) and the Wistar-Kyoto (WKY) rat. All rats in this study underwent a unilateral nephrectomy approximately 2 wk prior to the acute experiment. On the day of the acute experiment, renal denervation of the remaining left kidney was performed in one group of SHR (n = 6) and one group of WKY rats (n = 9). Another group of SHR (n = 6) and WKY rats (n = 10) had an innervated kidney. A control clearance period was taken, and then an L-NMMA (15 mg/kg bolus followed by 500 micrograms.kg-1.min-1 continuous infusion) infusion period followed in all four groups of rats. In the innervated SHR and WKY rats, the increases in fractional excretion of sodium (FENa) were 5.11 +/- 0.70 and 3.58 +/- 0.38%, respectively, with the infusion of L-NMMA and were associated with significant increases in fractional excretions of phosphate (FEPi; 18.18 +/- 5.33 and 6.34 +/- 2.29%, respectively), suggesting a reduction in proximal tubule reabsorption. In the SHRs with acute renal denervation, FENa was significantly increased by L-NMMA; however, FENa was significantly reduced (2.03 +/- 0.70%; P < 0.05) in comparison with innervated SHRs and was associated with no increase in FEPi (FEPi = -0.72 +/- 1.23%).(ABSTRACT TRUNCATED AT 250 WORDS)