Simultaneous quantification of the two major nitric oxide (NO) metabolites, nitrite and nitrate, was performed on mouse mesencephalic neurons in vitro, in basal conditions or after excitatory stimuli. Measurements were made using both extracellular medium and cytosolic extracts. Basal metabolite levels were stable up to 5 min. Depolarization by high levels of potassium promptly increased extracellular nitrite (270% of basal, peak at 10s) leaving intracellular levels unchanged. Glutamate receptor agonists caused a rapid increase in intracellular, but not extracellular, nitrite levels (240% of basal, peak at 30s). Intracellular nitrate levels raised slowly after addition of potassium (215% of basal at 1 min) or N-methyl-D-aspartate (230% of basal at 5 min), but not after quisqualate. The different dynamic variations of the two NO metabolites point to the specificity of the activated metabolism according to the type of excitatory stimulus.