DAF-2 is highly sensitive reagent for NO detection and determination of nitric oxide synthase activity. DAF-2, however, remains essentially nonfluorescent until it reacts with the nitrosonium cation (produced by spontaneous oxidation of nitric oxide) to form a fluorescent heterocycle, which becomes trapped in the cell’s cytoplasm. This sensitive fluorescent probe has been used to identify individual nitric oxide-producing neurons in brain slices, in mitochondria and in living plant cells. Spectral data: lambdaex 505nm; lambdaem 518nm in 0.1M Tris pH8.0.