We are pursuing a dual strategy for investigating the chemistry of nitric oxide as a biological signaling agent. In one approach, metal-based fluorescent sensors for the detection of NO in living cells are evaluated, and a sensor based on a copper fluorescein complex has proved to be a valuable lead compound. Sensors of this class permit identification of NO from both inducible and constitutive forms of nitric oxide synthase and facilitate investigation of different NO functions in response to external stimuli. In the other approach, we employ synthetic model complexes of iron sulfur clusters to probe their reactivity toward nitric oxide as biomimics of the active sites of iron sulfur proteins. Our studies reveal that NO disassembles the Fe-S clusters to form dinitrosyl iron complexes.