The comparative structural and functional features of supernatant and mitochondrial malate dehydrogenase from pig heart are being investigated. The possible functional significance of identical subunits is being investigated by physical methods (chemical and spectrophotometric). The structure of the active site is being studied in peptides covalently labelled in the active site by specific alkylation. The catalytic mechanism is under study with the use of a variety of kinetic and other physical methods.