The present studies examined the effect of agents that induce peroxisomal and mitochondrial β-oxidation on hepatic mitochondrial carnitine palmitoyltransferase (CPT) and peroxisomal carnitine acyltransferase [CPT(s) of Ramsay (1988) Biocehm. J. 249, 239-245; COT of Farrell & Bieber (1983) Arch . Biochem. Biophys. 222, 123-132 and Miyazawa, Ozasa, Osumi & Hashimoto (1983) J. Biochem. 94, 529-542]. In the first studies, high fat diets containing corn oil or fish oil were used to induce peroxisomal and mitochondrial enzymes. Rats were fed one of three diets for 4 weeks: (1) low fat, with corn oil as 11% of energy (kJ); (2) high fat, with corn oil as 45% of kJ; (3) high fat, with fish oil as 45% of kJ. At the end of 4 weeks, both mitochondrial CPT and peroxisomal CPT(s), exhibited increases in activity, immunoreactive protein, mRNA levels and transcription rates in livers of rats fed either high-fat diet compared to the low fat diet. Riboflavin deficiency or starvation for 48 h also increased the peroxisomal CPT(s) mRNA. A second set of studies used the plasticizer 2-(diethylhexyl)phthalate (DEHP), 0.5% clofibrate or 1% acetylsalicylic acid (fed for 3 weeks) to alter peroxisomal and mitochondrial fatty acid oxidation. With DEHP, the mitochondrial CPT and peroxisomal CPT(s) activity, immunoreactive protein, mRNA levels and transcription rate were all increased by 3-5-fold. The peroxisomal CPT(s) activity, immunoreactive protein, mRNA levels and transcription rate were increased 2-3-fold by clofibrate and acetylsalicylic acid, again similar to mitochondrial CPT. The results of the combined studies using both diet and drugs to cause enzyme induction suggest that the synthesis of the carnitine acyltransferases (mitochondrial CPT and peroxisomal CPT(s)) may be co-ordinated with each other; however, the co-ordinate regulatory factors have not yet been identified.