Distribution of rat liver serine: pyruvate aminotransferase between cytosol and mitochondria varies considerably with the dietary and hormonal state of animals. Feeding a high-protein diet or fasting the animals results in an increase in the enzyme activity of both fractions but more marked in the mitochondrial fraction. A low-protein diet exerts the reverse effect. A single administration of dibutyryl cyclic AMP causes a rapid elevation of the enzyme activity in both fractions, which is effectively prevented by cycloheximide, actinomycin D and cortisone. The activity in mitochondria increases with a lag of 2 h following injection of the nucleotide inducer, in contrast to the cytosol enzyme, which increases without any lag. Gel filtration and DEAE cellulose chromatography of the enzyme from both fractions revealed the similar pattern and some kinetic constants of these two types of the enzyme were not significantly different from each other. These results indicate that rat liver serine: pyruvate aminotransferase is synthesized in the extra-mitochondrial site and transfered to mitochondria.