There are several methods utilized for detection of nitric oxide (NO) and its various metabolites in biological systems. These include spectrophotometric analysis, high performance liquid chromatography, mass spectrophotometry, electro-paramagnetic resonance (EPR), and gas-phase chemiluminscence. Of these, the gas-phase chemiluminescence technique is perhaps the most sensitive, with a limit of detection in the femtomole range. Furthermore, as there are a number of commercially available NO analyzers, it is possible to directly compare results from different laboratories using this technique. Described in this unit is a group-specific derivatization approach with gas-phase chemiluminscence detection to profile NO metabolites in vivo.