Primary and secondary aliphatic nitroalkanes RR'CHNO2, react with microsomal cytochrome p-450 in the presence of dithionite leading to new complexes with a Soret peak at 455 nm. The formation of these complexes is inhibited completely by CO and partially by metyrapone. However, once formed, their exogenous ligand is not displaced by excess CO. By deoxycholate treatment they are transformed into 423-nm-absorbing cytochrome P-420 complexes, which are spectrally similar to the corresponding RR'CHNO2-derived myoglobin complexes. The 455-nm-absorbing complexes are equally produced from RR'CHNO2 reduction, microsomal NADPH-dependent oxidation of the corresponding hydroxylamine RR'CHNHOH, or interaction of the nitrosodimer (RR'CHNO)2 with reduced cytochrome P-450. All the reported results are consistent with the involvement of a new class of ligands of cytochrome P-450-Fe(II), which are the unstable aliphatic nitrosomonomers RR'CHNO, whose nitroso group is isoelectronic with dioxygen and whose stabilisation results from their strong binding to heme-Fe(II), thus explaining the observed inhibition of the hydroxylating function of cytochrome P-450.