A new mode of allosteric regulation of nucleic acid enzymes is described and shown to operate effectively with hammerhead ribozymes. In the "TRAP" design (for targeted ribozyme-attenuated probe), a 3' terminal "attenuator" anneals to conserved bases in the catalytic core to form the "off" state of the ribozyme. Binding of RNA or DNA to an antisense sequence linking the ribozyme and attenuator frees the core to fold into an active conformation, even though the antisense sequence itself does not interfere with the ribozyme. TRAP hammerheads based on the previously characterized HH8 ribozyme were shown to be activated more than 250-fold upon addition of the sense strand. RNA oligonucleotides were more effective activators than DNA oligos, consistent with the known relative helix stabilities (RNA-RNA > RNA-DNA). Oligonucleotides that directly paired with the attenuator gave up to 1760-fold activation. The magnitude of the activation was greater when the oligo was added prior to folding than if it was added during the cleavage reaction. The TRAP design requires no prior knowledge of (deoxy)ribozyme structure beyond identification of the essential core. Thus, this approach should be readily generalizable to other systems for biomedicine, sensor technology, and additional applications.