A chimeric RNA/DNA oligonucleotide was constructed to induce a sequence mutation in the rat factor IX gene, resulting in prolonged coagulation. Oligonucleotides were targeted to hepato-cytes in cell culture or in vivo by intravenous injection. Nucleotide conversion was both site-specific and dose-dependent. The mutated gene was associated in vivo with significantly reduced factor IX coagulant activity and a marked prolongation of the activated partial thromboplastin time. The results demonstrate that single base-pair alterations can be introduced in hepatocytes in situ by RNA/DNA oligonucleotides, suggesting a potentially powerful strategy for hepatic gene repair without the use of viral vectors.