Renal sympathetic nerve activity (RSNA) at rest and in response to changes in mean arterial pressure (MAP) were examined by measuring norepinephrine (NE) spillover rate across the kidney in 14 conscious rabbits. NE spillover rate was calculated from veno-arterial difference in plasma NE concentrations across the kidney corrected by the fractional extraction of infused tritiated NE. Renal NE spillover rate (mean +/- S.E.M.) at rest in 14 conscious rabbits was 14.7 +/- 0.9 ng/min. During sodium nitroprusside infusions at 10 and 20 micrograms/kg/min, renal NE spillover rate significantly increased by 55 and 101% in response to falls in MAP of 15 and 19% respectively. During phenylephrine infusion at 2 micrograms/kg/min, renal NE spillover rate significantly decreased by 40% in response to a 14% rise in MAP. The relative contribution of renal to total NE spillover rate decreased during sympathetic stimulation, while this ratio was unchanged during sympathetic inhibition. This study demonstrates that the renal NE spillover method is sufficiently sensitive and reliable to detect the responses in RSNA to physiological stimuli in conscious rabbits. The difference observed in the degree of stimulation of renal versus total NE spillover rate during hypotension produces confirmation that sympathetic responses are not uniformly distributed in the body.