It is known that several small RNAs including human and Xenopus signal recognition particle (SRP) RNA, U2 small nuclear RNA (snRNA) and 7SK RNAs are posttranscriptionally adenylated, whereas U6 snRNA and ribosomal 5S RNA are posttranscriptionally uridylated on their 3′ ends. In this study, we provide evidence that a small fraction of U6 snRNA and 5S ribosomal RNA molecules from human as well as Xenopus oocytes contain a single posttranscriptionally added adenylic acid residue on their 3′ ends. These data show that U6 snRNA and 5S rRNAs are posttranscriptionally modified on their 3′ ends by both uridylation and adenylation. Although the SRP RNA, 7SK RNA, 5S RNA, and U6 snRNA with the uridylic acid residue on their 3′ ends were readily uridylated, all these RNAs with posttranscriptionally added adenylic acid residue on their 3′ ends were not uridylated in vitro, or when U6 snRNA with 3′ AOH was injected into Xenopus oocytes. These results show that the presence of a single posttranscriptionally added adenylic acid residue on the 3′ end of SRP RNA, U6 snRNA, 5S rRNA, or 7SK RNA prevents 3′ uridylation. These data also show that adenylation and uridylation are two competing processes that add nucleotides on the 3′ end of some small RNAs and suggest that one of the functions of the 3′ adenylation may be to negatively affect the 3′ uridylation of small RNAs.