The posttranscriptional modification of messenger RNA precursors by base deamination can alter profoundly the function of the encoded proteins. Adenosine deaminases that act on RNA (ADARs) are capable of catalyzing the site-specific conversion of adenosine to inosine in pre-mRNA transcripts, thereby affecting coding potential of mature mRNAs. ADAR1 is a member of ADAR1 family and is abundantly expressed in the brain and ubiquitously present in most tissues of the mammals. Alternatively spliced forms of ADAR1 cDNA were cloned from the mouse liver and the localization of these splicing forms were examined under a fluorescent microscope by fusing them to EGFP. The cDNAs of these splicing forms were cloned into a baculovirus expression vector and the recombinant protein of these splicing forms were produced in insect cells and purified with Ni-NTA resin. The double-stranded RNA editing activity of the recombinant proteins were determined and compared. The results showed four major splicing forms of ADAR1 were naturally present in mouse liver and consisted of cDNAs of 3.5 kb, 3.6 kb, 1.8 kb and 2.0 kb in length, respectively. These three splicing forms were La, Lb, Sa and Sb forms and found to have two distinct translation initiator codons. Furthermore, the four splicing forms of ADAR1 protein were localized to either the cytoplasm or nucleus of NIH 3T3 cells as shown by fluorescent microscopy. The double-stranded RNA editing activity of the four splicing forms also differ significantly, with the Lb form possessing the highest editing activity. In summary, four major mouse ADAR1splicing forms are present naturally in the mouse liver with either cytoplasmic or nuclear localizations and widely varied editing activities, which indicate they might have different substrates and functions.