Aromatase cDNA clones were isolated from cDNA libraries of mouse hypothalamus, amygdala and ovary. Analysis of the nucleotide sequences of the 5' regions of the obtained cDNAs suggested that the mouse aromatase gene is tissue-specifically regulated by alternative exons 1. There were obvious differences between the 5' regions of the brain and ovary aromatase cDNAs, but no difference was found between the sequences of the hypothalamus and amygdala ones. We further isolated a mouse genomic DNA clone containing brain- and ovary-specific exons 1. The brain specific exons 1 and their promoters were highly homologous in the human and mouse aromatase genes. In contrast there were several differences in the sequences among the promoter regions of the ovary-specific exons 1 of the mouse, human and rat aromatase genes, significant homology between their sequences was also observed. The present results demonstrate that expression of the mouse aromatase gene is also tissue-specifically regulated through the use of alternative exons 1 and promoters, as reported for man.