The influence of clofibrate and di(2-ethylhexyl)phthalate on mitochondrial acetyl-CoA acetyltransferase (acetyl-CoA: acetyl-CoA C-acetyltransferase, EC 2.3.1.9), the rate-limiting ketogenic enzyme, which can be modified and inactivated by CoA, was investigated. In fed rats, both compounds induced a doubling of ketone bodies in the blood and, moreover, an increase by about 13% in the hepatic relative amount of the unmodified, i.e., the most active form of the enzyme (immunoreactive protein). This shift would account for an elevation of overall enzyme activity by about 5% only. Thus, the CoA modification of mitochondrial acetyl-CoA acetyltransferase did not explain the entire augmentation of ketone bodies. However, clofibrate and di(2-ethylhexyl)phthalate also increased the immunospecific protein and enzyme activity by approx. 2- and 3-fold, respectively. These effects were observed in liver, but not in several extrahepatic tissues.