The interaction between nitric oxide (NO) and the active site of ferric cytochrome P450 was studied by means of density functional theory (DFT), at the generalized gradient approximation level, and of the SAM1 semiempirical method. The electrostatic effects of the protein environment were included in our DFT scheme by using a hybrid quantum classical approach The active-site model consisted of an iron(III) porphyrin, the adjacent cysteine residue, and one coordinated water molecule. For this system, spin populations and relative energies for selected spin states were computed. Interestingly, the unpaired electron density, the HOMO, and the LUMO were found to be highly localized on the iron and in an appreciable extent on the sulfur coordinated to the metal. This provides central information about the reactivity of nitric oxide with the active site. Since the substitution of a molecule of HP by NO has been proposed as being responsible for the inhibition of the cytochrome in the presence of nitric oxide, we have analyzed the thermodynamic feasibility of the ligand exchange process. The structure of the nitrosylated active site was partially optimized using SAM1. A low-spin ground state was obtained for the nitrosyl complex, with a linear Fe-N-O angle. The trends found in Fe-N-O angles and Fe-N lengths of the higher energy spin states provided a notable insight into the electronic configuration of the complex within the framework of the Enemark and Feltham formalism Ln relation to the protein environment, it was assessed that the electrostatic field has significant effects on several computed properties: However, in both vacuum and protein environments, the ligand exchange reaction turned out to be exergonic and the relative orders of spin states of the relevant species were the same.