RIBOZYMES can be targeted to cleave specific RNAs(1-8), which has led to much interest in their potential as gene inhibitors(3,9,10). Such trans-cleaving ribozymes join a growing list of agents that stop the flow of genetic information(11,12). Here we describe a different application of ribozymes for which they may be uniquely suited. By targeted trans-splicing, a ribozyme can replace a defective portion of RNA with a functional sequence. The self-splicing intron from Tetrahymena thermophila(13) was previously shown to mediate trans-splicing of oligonucleotides in vitro(14,15). As a model system for messenger RNA repair, this group I intron was re-engineered to regenerate the proper coding capacity of short, truncated lacZ transcripts. Trans-splicing was efficient in vitro and proceeded in Escherichia coli to generate translatable lacZ messages. Targeted trans-splicing represents a general means of altering the sequence of specified transcripts and may provide a new approach to the treatment of many genetic diseases.