The controlled potential electrolysis of Ni(II) meso-tetraphenylporphyrin (Ni(II)TPP) gives at room temperature the corresponding metalloporphyrin π-cation radical [Ni(II)TPP]+ •. Upon freezing a solution of the π-cation radical to 77 K an internal electron transfer occurs to give [Ni(III)TPP]+. A discussion of the routes of electron transport in heme proteins is given, and the roles of metalloporphyrin π-cation radicals in electron transport is evaluated.