Summry—The nuclear encoded mitochondrial D‐3‐hydroxybutyrate dehydrogenase (BDH) is synthesized in the cytosal as a larger precursor. This membrane enzyme which requires lecithin for activity plays an essential role in energy metabolism as a ketone bodies‐converting enzyme. A cDNA clone of the rat liver enzyme encompassing an antigenic determinant peptide has been isolated after immunoscreening of a λ gt11 expression library. The nucleotide sequence of this 279‐base cDNA insert contains a single open reading frame of 93 amino‐acids, which represents about a third of the mature enzyme. Amino‐acid sequence analysis predicts a hydrophobic stretch of 29 amino‐acids long which probably functions as membrane anchor domain, or as an important region for the enzyme activation by phospholipid. By using this cDNA probe the BDH gene has been investigated at the mRNA level. There is only one mRNA (2‐kb size) for BDH whatever the studied tissue. The rat gene is differently expressed since its mRNA is already present in the foetus liver while the BDH polypeptide amount is low and its enzymatic activity is not detectable even in the late stage of foetal development. The mRNA content is higher in the liver than in extrahepatic tissues. Adrenalectomy and ovariectomy increase liver mRNA content and polypeptide level, as well as activity of BDH. These effects are totally or partially abolished by corticosterone and estradiol treatments respectively. In addition, a 15‐day hyperlipidic diet stimulates BDH gene expression. Present results show that the gene expression of this mitochondrial enzyme is modulated through development and hormonal and metabolic condit