A 2.1 kb cDNA clone for rat transketolase was isolated from rat liver ${lambda}gt11$ cDNA library and its sequence was determined. The predicted rat transketolase (655 amino acids with $M_r$ 71,186) is highly similar (92%) to that of the human enzyme except that it contains an extra 32 amino acids at its N-terminus. Although it is less similar (<27%) to transketolases from non-mammalian species, the functional motifs such as the catalytic sites and thiamine binding domain are well conserved in the rat enzyme. Southern blot analysis of genomic DNA verified that transketolase appears to be derived from a single gene. Immunoblot and Northern blot analyses suggested that hepatic transketolase was activated pretranslationally by a 2.1-fold while little change was observed in brain enzyme, indicating a tissue-specific pretranslational activation during postnatal development.