The discoveries that nitric oxide serves important roles in mammalian bioregulation and immunology have stimulated intense interest in the chemistry and biochemistry of NO and derivatives such as metal nitrosyl complexes. Also of interest are strategies to deliver NO to biological targets on demand. One such strategy would be to employ a precursor which displays relatively low thermal reactivity but is photochemically active to give NO. This proposition led the authors to investigate photochemical properties of metal nitrosyl complexes such as the iron-sulfur-nitrosyl Roussin cluster anions Fe 2 S 2 (NO) 4 2− and Fe 4 S 3 (NO) 7 − as well as metalloporphyrin nitrosyls including ferriheme complexes (with M. Hoshino of the Institute of Physical and Chemical Research, Japan) and nitrosyl nitrito complexes of ruthenium porphyrins Ru(P)(ONO)(NO). Continuous and flash photolysis studies of these compounds are reviewed here as are studies (with D.A. Wink and J.B. Mitchell of the Radiation Biology Branch of the US National Cancer Institute) using metal nitrosyl photochemistry as a vehicle for delivering NO to hypoxic cell cultures in order to sensitize γ-radiation damage.