beta-Alanine-oxoglutarate aminotransferase from rat liver was mainly distributed in the mitochondrial fraction, while beta-ureidopropionase, the last one of uracil-metabolizing enzymes to beta-alanine, was predominantly distributed in the cytosolic fraction. When rat liver mitochondria were separated into submitochondrial fractions, beta-alanine-oxoglutarate aminotransferase was localized in the mitochondrial matrix. beta-Alanine was transported into mitochondria with time and the influx of beta-alanine into the matrix was 0.47 nmol/mg of mitochondrial protein at 2 min after incubation.