Incubation of photosystem II, PSII, membranes with NO for a few minutes results in the reversible elimination of the electron paramagnetic resonance (EPR) signal II from the oxidized Tyr Y(D)., presumably due to the formation of a weak Tyr Y(D).-NO complex [Petrouleas, V., & Diner, B. A. (1990) Biochim. Biophys. Acta 1015, 131-140]. Illumination of such a sample at ambient or cryogenic temperatures produces no new EPR signals. If, however, the incubation with NO is extended to the hours time range, illumination induces an EPR signal with resolved hyperfine structure in the g = 2 region. The signal shows the typical features of an immobilized iminoxyl radical (> C=NO.) with hyperfine values A(parallel) = 44 G, A(perpendicular) = 22 G, and A(iso) = 29.3 G. The following observations suggest that the iminoxyl signal is associated with PSII: (a) the signal results from an immobilized species at room temperature probably associated with a membrane-bound component, (b) the abundance of the signal is (sub)stoichiometric to PSII, (c) the signal is light-induced, (d) some of the treatments that affect PSII (Tris, Ca2+ depletion, high-salt wash) severely diminish the size of the signal, and (e) the development of the signal correlates with the release of Mn. In addition, the following observations suggest that the iminoxyl signal results from an interaction of Y(D). with NO: (a) the evolution of the signal correlates with the loss in reversibility of the Tyr Y(D).-NO interaction and (b) the size of the signal correlates with the initial amount of oxidized Tyr Y(D). It is accordingly proposed that during the incubation with NO, a weak Tyr Y(D).-NO complex is rapidly formed and is then slowly converted to a tyrosine-nitroso adduct. Light-induced oxidation of the latter produces the iminoxyl radical. The nitrosotyrosine is expected to have an oxidation potential significantly lower than the parent tyrosine and can act as an efficient electron donor in PSII even at cryogenic temperatures. It is probably this lowered redox potential of the tyrosine Y(D) that explains the release of Mn concomitant with the formation of the nitroso species.