Mouse adrenocorticotropin receptor (ACTH-R/MC2R) messenger ribonucleic acid (mRNA) is expressed predominantly in the adrenal gland and, to a lesser extent, in adipose tissue. In this study, we found a novel 135-bp exon 1 (exon 1f) of the ACTH-R gene transcribed in mouse adipose tissue by RNA ligase-mediated rapid amplification of cDNA ends, which was located 1.4 kb downstream in the genome of previously-reported exon 1 (exon 1a) transcribed in the adrenal gland. The novel promoter region, 1.4 kb upstream of exon 1f contained three CCAAT boxes. RT-PCR analysis revealed that ACTH-R mRNA from adipose tissue and differentiated 3T3-L1 adipocytes exclusively contained exon 1f. Thus, the promoter region flanking to exon 1f is thought to be essential for adipose tissue, while that flanking to exon 1a is specific for the adrenal gland. A search for a similar sequence of mouse ACTH-R exon 1f and its flanking region in the human genome sequence database of GenBank Human Genome Resources did not reveal such a sequence in the region of the human ACTH-R gene. This may explain the absence of ACTH-R expression in human adipose tissue.