1. A large proportion of sympathetic preganglionic neurones contain nitric oxide synthase. The purpose of this study was to determine the effects of facilitation and inhibition of nitric oxide synthesis within the lower thoracic spinal cord (which contains the majority of renal preganglionic neurones) on renal sympathetic nerve activity (rSNA). 2. In anaesthetized rabbits, rSNA was recorded before and after intrathecal injection (50 microliters of 0.5 M solution) of either L‐arginine, a precursor of nitric oxide, or N omega‐nitro‐L‐arginine methyl ester (L‐NAME), an inhibitor of nitric oxide synthase, into the lower thoracic spinal cord. Spinal cord sections were also stained for the presence of NADPH diaphorase, a marker of nitric oxide synthesizing neurones. 3. A high density of NADPH diaphorase‐containing neurones was found within the intermediolateral cell column of the lower thoracic spinal cord. 4. Intrathecal injection of L‐arginine and L‐NAME resulted in a large increase (113 +/‐ 25%) and decrease (43 +/‐ 8%), respectively, in rSNA. In contrast, injection of the inactive isomers D‐arginine and D‐NAME had no significant effect on rSNA. 5. The results indicate that endogenous nitric oxide in the lower thoracic spinal cord (1) has a potent excitatory action on renal sympathetic preganglionic neurones, and (2) helps to maintain the tonic activity of renal sympathetic nerves under resting conditions.