The formation of nitric oxide (NO) radicals was demonstrated by electron paramagnetic resonance spectroscopy in the rat during varying degrees of brain ischemia. Diethyldithiocarbamate and Fe-citrate were used as in vivo spin-trapping reagents. The signal of NO spin adducts increased in accordance with the degree of ischemic insults. The formation of NO radicals was inhibited by NG-nitro-L-arginine methyl ester.