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Mitochondrial porin or VDAC (voltage-dependent anion channel) is the intracellular aqueous pore which allows the very high permeability of the outer mitochondrial membrane (Colombini, 1979). It was shown to be present in every tissue investigated and in many organisms with approximately the same functional features, the same molecular size and a similar quantitative expression (De Pinto et al, 1987). The primary structure of porins from various sources has been determined. A great deal of interest is nowadays focussed in the structural arrangement of the polypeptide chain(s) of porin and its/their role in the properties of the channel. In the first part of this paper we give further experimental supports to a sixteen-stranded beta-barrel model of the transmembrane topology of the mammalian mitochondrial porin we have previously published (De Pinto et al, 1991b). The results reported here concern with the localization of the hydrophobic region responsible for the DCCD-binding and the characterization of porin cysteines with respect to their position in the channel unit. At the end our model and other models for mitochondrial porin appeared in the literature are discussed and compared to the bacterial porin structures determined by crystal analysis.