A soluble enzyme preparation which catalyzes transamination between 3:5 diiodo-L-tyrosine and alpha-ketoglutarate has been obtained from rat kidney mitochondria disintegrated by repeated freezing and thawing or sonic treatment. In the presence of added pyridoxal phosphate and alpha-ketoglutarate, crude preparations deaminated this diiodotyrosine as well as monoiodo-L-tyrosine and L-tyrosine. Judging from Km values, the enzyme preparation was most active when the diiodotyrosine was the substrate. The preparation lost about 80% of its activity after heating at 56 C for 5 minutes. Anaerobiosis did not inhibit transamination activity but did suppress ability to oxidize triiodo-Ltyronine, a property which was readily demonstrable under aerobic conditions. It is concluded that the preparation contained at least 2 different enzyme systems: one which catalyzed the oxidative deamination of triiodo- L-thyronine without added cofactors and another which, in the presence of pyridoxal phosphate, catalyzed transamination between L-tyrosine or its diiodo and monoiodo derivatives and alpha-ketoglutarate.