Water-soluble iron(III) porphyrin and ferrihemoproteins (methemoglobin, metmyoglobin, oxidized cytochrome c, and catalase) associate with NO to yield the nitric oxide adducts. The equilibrium constants for association of ferrihemoproteins and NO are 1 order of magnitude larger than that of the water-soluble iron(III) porphyrin which is free from protein, suggesting that the proteins offset the forward and backward reaction rates in the equilibrium reactions. Nanosecond laser photolysis studies of the nitric oxide adducts of metmyoglobin, oxidized cytochrome c, and catalase, (NO)MbIII, (NO)CytIII, and (NO)CatIII, have been carried out. The transient detected after laser flash photolysis of (NO)CatIII is identified as CatIII. However, the transients observed for (NO)MbIII and (NO)CytIII at 50 ns after laser pulsing are ascribed to MbIIItr and CytIIItr, respectively, with the absorption spectra different from those of uncomplexed MbIII and CytIII. In particular, the absorption spectrum of CytIIItr markedly differs from that of the uncomplexed CytIII. The species MbIIItr and CytIIItr are found to change to MbIII and CytIII, respectively, within a few microseconds. The quantum yields for the photodissociation of NO from nitric oxide adducts of ferrihemoproteins are 1 order of magnitude less than that from the NO adduct of the water-soluble iron(III) porphyrin, probably due to fast geminate recombination reaction of NO and ferrihemoprotein in a heme pocket. The photochemistry of the nitric oxide adducts of hemoproteins and water-soluble iron(II) porphyrin is also described on the basis of laser phosolysis studies. © 1993, American Chemical Society. All rights reserved.