A cell-permeable fluorogenic dye useful for the detection of reactive oxygen species (ROS) and nitric oxide (NO) and for the determination of the degree of overall oxidative stress. The fluorescence of DCFDA indicates oxidation by hydrogen peroxide, peroxynitrite or hydroxyl radical. Superoxide anions can also contribute to DCFDA oxidation albeit at a lesser degree. It measures hydroxyl, peroxyl and other reactive oxygen species (ROS) activity within the cell. After diffusion in to the cell, DCFDA is deacetylated by cellular esterases to a non-fluorescent compound, which is later oxidized by ROS into the highly fluorescent 2’,7’-dichlorofluorescein (DCF). Spectral data: lambdaex=504nm, lambdaem=524nm in 0.1 M Tris pH 8.0 (esterase).