The structural characterizations of the Fe environments in the NO adducts of horse heart ferric (Fe-III) and ferrous (Fe-II) nitrosylmyoglobin (MbNO) have been achieved by multiple-scattering (MS) analyses of XAFS data obtained from frozen aqueous solutions. For Mb(II)NO, the MS analysis resulted in an averaged Fe-N-p (pyrrole) distance of 2.00 Angstrom, an Fe-N-epsilon (imidazole) distance of 2.05 Angstrom, an Fe-N-NO distance of 1.75 Angstrom, and an Fe-N-O angle of 150 degrees. For comparison, the MS analysis of the XAFS data of [Fe-II(TPP)(NO)] (TPP = tetraphenylporphyrinato) resulted in bond lengths of 2.01 Angstrom for Fe-N-p and 1.74 Angstrom for Fe-N-NO and an Fe-N-O bond angle of 155 degrees. The Fe-N-NO and N-O distances obtained from the analysis of Mb(II)NO are in good agreement with those determined crystallographically for [Fe-II(TPP)(NO)] with and without 1-methylimidazole (1-MeIm) as the sixth ligand. The MS analysis of Mb(III)NO yielded an average Fe-N-p distance of 2.00 Angstrom, an Fe-N-epsilon distance of 2.04 Angstrom, an Fe-N-NO distance of 1.69 Angstrom, and an Fe-N-O angle of 180 degrees. These bond lengths and angles are consistent with the crystal structures of model complexes [Fe-III(TPP)(NO)(OH2)](+) and [Fe-III(OEP)(NO)](+) (OEP = octaethylporphyrinato). The final XAFS R values were 14.5, 14.6, and 12.7% for Mb(III)NO, Mb(II)NO, and [Fe-II(TPP)(NO)], respectively.