Nitric oxide (NO) in brain has been implicated in neuronal regulatory processes and in neuropathologies. Previously we showed that NO modified quinpirole induced yawning, a behavioral measure of dopamine (DA) D3 receptor activation in rats. The aim of this study was to characterize the effect of nitro L arginine methyl ester HCl (NAME) and L arginine HCl on reactivity of rats to the DA D1 receptor agonist SKF 38393 and DA D1 antagonist SCH 23390 in intact and neonatal6-hydroxydopamine (6 OHDA) lesioned rats (134 mug of base ICV at 3rd dayafter birth). L arginine HCl (300 mg/kg IP) increased the oral activity response in 6 OHDA lesioned rats, like SKF 38393, and induced catalepsy in intact control rats, like SCH 23390. In contrast, NAME had no effect on oral activity or catalepsy, but fully attenuated SKF 38393 induced oral activity. These findings indicate that L arginine HCl has no apparent effect at the DA D1receptor, but that NAME is effective in attenuating a DA D1 agonist - induced effect. Consequently NO may be an intracellular second mesenger for supersensitized receptors associated with DA D1 agonist induced oral activity.ity.