The effects of nitric oxide synthase (NOS) inhibition on extracellular glutamate release were investigated in rats during global brain ischemia and reperfusion (IR) using cerebral microdialysis. A dialysis probe was inserted into the hippocampus of anesthetized rats. Forebrain ischemia was produced by hypotension and occlusion of both carotid arteries. After 15 min, brain flow was restored for 60 min. Time-dependent changes in the dialysate glutamate concentration were analyzed with HPLC in both control rats and those treated with N omega-nitro-L-arginine methyl ester 30 min prior to ischemia. The data show that the NOS inhibitor did not prevent glutamate release from hippocampus during ischemia. Inhibition of NOS also enhanced glutamate release during reperfusion resulting in dialysate concentrations up to 10 times higher than control values.