A high level of plasmid DNA expression in rat liver can be achieved by the rapid injection of a large volume of a naked DNA solution into the tail vein, called the 'hydrodynamics-based procedure.' The preparation of PCR-amplified DNA fragments is easier than that of naked DNA. In this paper we evaluated the effects of expressing the erythropoietin (Epo) gene in the rat liver by injecting fCAGGS-Epo, an Epo-expressing PCR-amplified DNA fragment, via the tail vein. After injection of 5 pmol fCAGGS-Epo (10 microg) or pCAGGS-Epo (18.4 microg), plasmid DNA, the serum Epo levels peaked at week 1, then persisted for at least 12 weeks. Transgene-derived Epo secretion resulted in significant erythropoiesis. These results demonstrated that transfer of PCR-amplified DNA fragments into the rat liver via rapid tail vein injection can be achieved. This method may provide a useful means for studying the physiologic function of a putative gene.