Studies in this laboratory are concerned with elucidating chemical mechanisms relevant to roles played by nitrogen monoxide and other NO_x species in mammalian bioregulation and immunology. Dynamic photochemical techniques have proved very useful in probing these mechanisms. We also have interest in strategies to deliver NO to biological targets upon demand for such goals as the sensitization of γ-radiation damage in hypoxic tissue. One such strategy would be to employ a precursor that displays relatively low thermal reactivity but is photochemically active to release NO. This proposition has led us to investigate the flash and continuous photolysis kinetics of a number of different nitrosyl complexes and other NO and NO_x precursors such as metal nitrito complexes. The systems probed include several ruthenium salen and porphyrin nitrosyls, manganese(III) and chromium(III) nitrito complexes, ferri- and ferro-heme models and proteins, and iron-sulfur-nitrosyl clusters known as the Roussin's anions (e.g. Fe₂S₂(NO)₄²⁻) and esters (e.g., Fe₂(μ-SR)₂(NO)₄, where R is an organic functional group). An overview of these studies is presented.