We have carried out an in vitro selection to obtain an allosteric hairpin ribozyme, which has cleavage activity in the presence of an exogenous short oligonucleotide as a regulator. Random sequences were inserted in a region corresponding to the hairpin loop of the ribozyme. After 12 rounds of selection, DNA templates were cloned. Of a total of 34 clones, 18 contained the same sequence, and the obtained hairpin ribozymes showed the cleavage activity specifically in the presence of the regulator oligonucleotide. All of the clones contained sequences complementary to the regulator oligonucleotide. The ribozymes with high cleavage activities gained characteristic hairpin loops at the random domain, which were similar to each other. In the absence of the oligonucleotide, the loop domain within the allosteric ribozyme probably forms a slipped hairpin loop, and the complementary sequence, with the regulator oligonucleotide located at the single stranded loop, would allow easy access of the oligonucleotide. The binding of the regulator oligonucleotide triggers a structural change of the hairpin loop to form an active conformation. Furthermore, we constructed an allosteric hammerhead ribozyme by introducing the characteristic hairpin loop. The modified hammerhead ribozyme was also changed to an allosteric ribozyme, which was activated by the addition of the regulator oligonucleotide. The characteristic hairpin loop, which was proved to be regulated by an exogenous oligonucleotide in this report, may be used to control RNA functions in various fields.