Nitric oxide has been implicated in N-methyl-D-aspartate (NMDA)-mediated damage in vitro; however, its role in excitotoxic damage in vivo is not clear. In the present study we evaluated the histopathological and hemodynamic consequences of intrastriatal injections of various doses of NMDA and determined the effects of nitric oxide synthase inhibition on these changes. NMDA was injected into the striatum at doses of 50, 150, and 300 nmol with or without N omega-nitro-L-arginine methyl ester (L-NAME; 100 micrograms, locally). Three days following injections histopathological assessment was performed by morphometric analysis of the lesion area in multiple sections taken from the anterior to the posterior borders of the lesion. In animals injected with 150 and 300 nmol of NMDA (+/- L-NAME), local CBF (lCBF) was determined 30 min following injections using 14C-iodoantipyrine autoradiography. All NMDA-treated animals showed a well-demarcated lesion extending beyond the injection site. The volume of the lesion correlated significantly with the NMDA dose injected. The effects of L-NAME on lesion size were dependent on the dose of the NMDA. The lesion induced by 50 nmol of NMDA was not affected by L-NAME. With a dose of 150 nmol of NMDA, L-NAME induced a 43% increase in lesion volume. In contrast, a 38% decrease in lesion size was observed in animals treated with 300 nmol of NMDA combined with L-NAME. At a dose of 150 nmol, NMDA induced a significant elevation in lCBF, which was restricted to regions close to the injection site including the center areas of the anterior and middle striatum. The increase in lCBF observed with 150 nmol of NMDA was significantly attenuated in the NMDA + L-NAME-treated group. The lCBF changes induced by 300 nmol of NMDA were not significantly different from those in the 150-nmol group; however, the extent of the regions involved was larger. The increases in lCBF were observed in all striatal regions including the central and peripheral areas. L-NAME did not have a significant effect on the lCBF changes induced by NMDA at a dose of 300 nmol. These data suggest that in vivo the involvement of nitric oxide in NMDA toxicity depends on the NMDA dose and on the participation of hemodynamic mechanisms secondary to NMDA exposure.