Kidney-targeted gene transfer is expected to revolutionize the treatment of renal diseases. Recently, we demonstrated that naked plasmid deoxyribonucleic acid (DNA) can be transferred into renal interstitial fibroblasts near the peritubular capillaries (PTCs) in normal rats, by retrograde injection into the renal vein with the renal vein and artery clamped. The PTC network is a main target of kidney transplant rejection and of progressive tubulointerstitial fibrosis, which typifies all progressive renal diseases. We retrogradely injected a lacZ expression plasmid in Ringer's solution into the renal vein of rats using a 24-gage catheter. We detected lacZ expression exclusively in the interstitial fibroblasts near the PTCs of the kidney by immunoelectron microscopy. Nephrotoxicity from the gene transfer was not apparent. We then used a rat erythropoietin (Epo) expression plasmid vector pCAGGS-Epo in a reporter assay. We obtained maximal Epo expression when the DNA solution was injected within 5 s in a volume of 1.0 mL. We detected transgene-derived Epo messenger ribonucleic acid by reverse transcriptase polymerase chain reaction only in the kidneys receiving pCAGGS-Epo. In this article, protocols for naked plasmid DNA transfer into rat kidney using this hydrodynamics-based transfection method and the immunoelectron microscopic technique to determine the lacZ gene transfer site are described in detail.