In the conscious rabbit muscimol (1-10 nmol/side) was microinjected into the hypothalamic paraventricular nucleus to inhibit neuronal function and the acute changes in mean arterial pressure (MAP), heart rate and renal sympathetic nerve activity (RSNA) were monitored. Muscimol (1 nmol) had no effect on the cardiovascular variables, as was the case with vehicle. However, muscimol (10 nmol) elicited a significant increase in RSNA of 184+/-40% and a reduction in heart rate of 49+/-12 beats/min but no change in MAP. The effect of blocking endogenous glutamatergic inputs with the glutamate antagonist, kynurenate (25 nmol), into the PVN was also examined. Kynurenate elicited an increase in RSNA of 35+/-9% with no significant change in MAP or HR. The results suggest that muscimol inhibits a tonically active inhibitory influence on RSNA arising from the PVN in the conscious rabbit. A glutamatergic input into the PVN appears to contribute to the tonic activation of this inhibitory influence.