Both nitric oxide (NO) and GABA are known to provide inhibitory inputs to the paraventricular nucleus (PVN) of the hypothalamus and are involved in the control of sympathetic outflow. The purpose of the present study was to examine the interaction of NO and GABA in the regulation of renal sympathetic nerve activity in rats. The responses of renal nerve activity, blood pressure, and heart rate to microinjection of sodium nitroprusside (SNP), an NO donor, into the PVN were measured in the presence and absence of blockade of the GABA system (bicuculline; 2 nmol). Microinjection of SNP (50, 100, and 200 nmol) into the PVN elicited significant decreases in renal nerve discharge, arterial blood pressure, and heart rate, reaching -36.4 +/- 9.7%, -11 +/- 5 mmHg, and -34 +/- 14 beats/min, respectively, at the highest dose. These responses were eliminated by blockade of the GABA system. Conversely, microinjection of Nomega-nitro-L-arginine methyl ester (L-NAME; 50, 100, and 200 nmol) elicited significant increases in the renal sympathetic nerve discharge, arterial blood pressure, and heart rate, reaching 88.9 +/- 16.6%, 9 +/- 1 mmHg, and 29 +/- 9 beats/min, respectively, at the highest dose. These sympathoexcitatory responses were masked by prior blockade of the GABA system with bicuculline. The sympathoexcitatory effect of L-NAME was also eliminated by activation of the GABA system with muscimol. In conclusion, our data indicate that the inhibitory effect of endogenous NO within the PVN on the renal sympathetic nerve activity is mediated by GABA.