The fluorescence of 2',7'-dichlorofluorescin (DCF) was measured in acutely dissociated rat cerebellar neurons as a mean of estimating the formation of reactive oxygen species (ROS). N,N-Diethyldithiocarbamate, an inhibitor for superoxide dismutase, reduced the intensity of DCF fluorescence in a dose-dependent fashion at concentrations of 30 nM to up to 10 mu M. N-Ethylmaleimide, an inhibitor for glutathione peroxidase, augmented the DCF fluorescence in a dose-dependent manner at concentration of 10 mu M to 1 mM while 3-amino-1,2,4-triazole, an inhibitor for catalase, did not change the fluorescence intensity even at concentrations as high as 1 mM. Hydrogen peroxide, applied externally at concentrations between 3 mu M and 3 mM, augmented the fluorescence in a dose-dependent fashion. These results suggest the possibility that the DCF fluorescence may be useful in estimating the intracellular content of hydrogen peroxide of mammalian brain neurons.