Nitric oxide (NO) reacts with catecholamines resulting in their deactivation. In the present study using the perfused mesenteric arterial bed as a model of the sympathetic neuroeffector junction, the NO synthase (NOS) inhibitor N-nitro-L-arginine methyl ester (L-NAME) resulted in the enhancement in the periarterial nerve stimulation induced increase in perfusion pressure and norepinephrine overflow while decreasing Neuropeptide Y (NPY) overflow. These changes were prevented by L-arginine, demonstrating that the effects of L-NAME were specific to the inhibition of NOS. Based on the fact that norepinephrine acts on pre-junctional ₂-adrenoceptors to inhibit the evoked release of sympathetic co-transmitters, we carried out experiments in the presence of the ₂-adrenergic receptor antagonist yohimbine to investigate the possibility that the decrease in NPY observed in the presence of L-NAME was due to the increase in bioactive norepinephrine acting upon its autoreceptor. Periarterial nerve stimulation in the presence of both L-NAME and yohimbine prevented the previously observed decrease in NPY indicating that the cause of this decrease was, as predicted, due to ₂-adrenoceptor activation. The periarterial nerve stimulation induced increase of norepinephrine overflow was greater in the Spontaneously Hypertensive Rat (SHR) compared to normotensive rats. In contrast to what was observed in the isolated perfused mesenteric arterial bed obtained from normotensive animals, inhibition of NOS did not result in further increase in the overflow of norepinephine or in a subsequent decrease in NPY. These results demonstrate that, in addition to being direct vasodilator, NO, by deactivating norepinephrine, can modulate sympathetic neurotransmission and that this modulation is altered in the SHR.