We examined the modulation of nitric oxide production in vivo by measuring levels of nitrite (NO2-) and nitrate (NO3-) in the dialysate of the cerebellum in conscious rats, by using an in vivo brain microdialysis technique. The levels of both NO2- and NO3- were decreased by the intraperitoneal injection of NG-nitro-L-arginine methyl ester, an inhibitor of nitric oxide synthase, whereas NG-nitro-D-arginine methyl ester had no effect. L-Arginine by itself increased NO2- and NO3- levels and diminished the reduction of their levels caused by NG-nitro-L-arginine methyl ester. Direct infusion of L-glutamate, N-methyl-D-aspartate, or KCl into the cerebellum through a dialysis probe resulted in an increase in NO2- and/or NO3- levels. The effects of N-methyl-D-aspartate and KCl were dependent on extracellular calcium. Furthermore, the stimulatory effects of L-glutamate and N-methyl-D-aspartate were inhibited by NG-nitro-L-arginine methyl ester and (+/-)-3-(2-carboxypiperazin-4-yl)propyl-1-phosphonic acid (CPP), an N-methyl-D-aspartate receptor antagonist. These results suggest that NO2- and NO3- levels may be related to nitric oxide production in vivo.