Quinolinic acid (QUIN) is an endogenous excitotoxin acting on N-methyl-D-aspartate (NMDA) receptors, that leads to neurotoxic damage resembling the alterations observed in Huntington's disease. Two major end-points of QUIN induced neurotoxicity are both circling behavior (CB) and lipid peroxidation (LP). Recently, nitric oxide (NO) has been implicated as a mediator of cell injury in some neurological disorders, thus, NO as a free radical might be involved in QUIN-induced neurotoxicity and oxidative stress. In the present study we evaluated the possible role of NO on QUIN-induced neurotoxicity, by measuring nitric oxide synthase activity (NOS), before and after QUIN-induced damage and by evaluating the effect of NOS inhibition on acute QUIN-induced CB and LP. Rats were striatally microinjected with QUIN (240 nmol/microl). QUIN administration increased NOS activity by 327% as compared to control values and this enhancement was inhibited by i.v. pretreatment with a NOS inhibitor the N(G)-nitro-L-arginine methyl ester (L-NAME) (10 mg/kg). QUIN-induced CB was also attenuated by pretreatment of rats with 1, 5, 10 and 15 mg/kg of L-NAME by -37, -55, -62 and -74% vs QUIN respectively. Similarly, L-NAME also reduced by 32% the QUIN-induced LP. These findings suggest that enhanced NOS activity may participate in QUIN-induced neurotoxicity and oxidative stress.