The simultaneous oxidation of malate and of glycine was investigated in pea (Pisum sativum) leaf mitochondria. Adding malate to state 4 glycine oxidation did not inhibit, and under some conditions stimulated, glycine oxidation. State 4 oxygen uptake with glycine is restricted because of the control exerted by the membrane potential but reoxidation of NADH by oxaloacetate reduction can still occur. Thus, malate addition stimulates glycine metabolism by producing oxaloacetate. The malate dehydrogenase (EC 188.8.131.52) enzyme fraction remote from glycine decarboxylase (EC 184.108.40.206) oxidizes malate whereas that closely associated with it produces malate, i.e. they function in opposite directions. It is shown that these opposing directions of malate dehydrogenase activity occur within the same mitochondrial matrix compartment and not in different mitochondrial populations. It is concluded that metabolic domains containing different complements of mitochondrial enzymes exist within the one mitochondrial matrix without physical barriers separating them. The differential spatial organization within the matrix may account for the previously reported limited access of some enzymes to the respiratory electron transport chain. The implications for leaf mitochondrial metabolism are discussed.