Receptor-mediated gene transfer has been used to introduce genes into tissues of animals in vivo. The genes introduced by this approach have been transiently expressed at low levels in animal tissues. High levels of expression, for longer periods, have been attained by the induction of cell division (i.e., partial hepatectomy) or disruption of lysosomal degradation of the DNA. We have studied the correlation of specific structural features on the DNA/ligand complexes with their ability to efficiently introduce DNA into the livers of intact animals. A chimeric gene containing the phosphoenolpyruvate carboxykinase gene promoter (nucleotides -460 to +73) linked to the structural gene for human factor IX (PEPCK-hFIX gene) was condensed with galactosylated poly(L-lysine) by titration with NaCl, resulting in complexes of defined size (10-12 nm in diameter) and shape. The PEPCK-hFIX gene complex was injected into the caudal vena cava of adult rats and the conjugated DNA was specifically targeted to the livers of the animals; no detectable DNA was noted in other tissues. The plasmid containing the PEPCK-hFIX gene was found as an episome in the livers of the rats 32 days after injection of the DNA complex. Human factor IX DNA, mRNA, and functional protein were detected up to 140 days after administration of the DNA complex (the duration of the experiment). Transcription from the PEPCK promoter could be induced over the entire course of the experiment by feeding the rats a high-protein, carbohydrate-free diet. We conclude that the structure of the DNA/ligand complexes is of key importance for the successful introduction of genes into the tissues of animals by receptor-mediated endocytosis.