In mammalian cells, genetic instructions are usually revised by RNA splicing before they are translated to proteins. Here we demonstrate that a trans–splicing group I ribozyme can be employed to intentionally modify the sequence of targeted transcripts in tissue culture cells. By analyzing the ribozyme reaction products, we demonstrate that targeted trans–splicing can proceed in murine fibroblasts with high fidelity, providing direct evidence that ribozymes function as anticipated in a therapeutically relevant setting. Tram–splicing is not very specific however, and the ribozyme reacted with and tagged a variety of cellular transcripts with its 3^′ exon sequence. RNA tagging provides a unique approach to study RNA catalysis in mammalian cells. Such analysis should facilitate the logical development of safe, therapeutic ribozymes that can repair mutant RNAs associated with a variety of inherited diseases.