Double-stranded RNA adenosine deaminase (dsRAD), previously called the double-stranded RNA (dsRNA) unwinding/modifying activity, modifies adenosines to inosines within dsRNA. We used ribonuclease U2 and a mutant of ribonuclease T1 to map the sites of modification in several RNA duplexes. We found that dsRAD had a 5' neighbor preference (A = U > C > G) but no apparent 3' neighbor preference. Further, the proximity of the strand termini affected whether an adenosine was modified. Most importantly, dsRAD exhibited selectivity, modifying a minimal number of adenosines in short dsRNAs. Our results suggest that the specific editing of glutamate receptor subunit B mRNA could be performed in vivo by dsRAD without the aid of specificity factors, and support the hypothesis that dsRAD is responsible for hypermutations in certain RNA viruses.