Muscle-type carnitine palmitoyltransferase I (M-CPTI) is the key enzyme for fatty acid beta-oxidation in heart and skeletal muscles and in adipose tissue. So far, M-CPTI mRNA has been detected in white adipocytes from epididymal fat pads of rats and humans, but not in mouse adipocytes. To characterize the gene expression of M-CPTI in mice, we isolated the genomic DNA encoding mouse M-CPTI and determined its transcription initiation site. As a result, the mouse M-CPTI gene seemed to have multiple initiation sites, as in the case of the rat and human genes. Furthermore, the conserved nucleotide sequence of the response element for peroxisome proliferators was shown to exist in the upstream of the mouse gene as in that of the rat and human genes. From these observations, we suggest that the anomalous expression of M-CPTI in mouse adipocytes reported previously may be regulated by factors other than peroxisome proliferators. Previously, we reported that there were transcripts containing regions of both CK/EK-beta and M-CPTI genes in humans. In this study, we found that such transcripts also exist in rodents and that the amounts of the transcripts containing regions of both of these genes did not depend on the expression level of CK/EK-beta.