Gene transfer using retroviral vectors requires cell replication for insertion of the DNA provirus. Since the mitotic index of the mammalian kidney is very low, renal tubular cell replication was induced in adult rats as part of a regenerative response to the nephrotoxic injury induced by an intraperitoneal injection of folic acid. At 48 h, at the time of maximum 3H-thymidine incorporation, the left kidney was directly injected with a suspension of the Psi2 BAG retrovirus which transduces the beta-galactosidase gene. At 1-7 weeks after virus administration the left kidney was harvested. Using the polymerase chain reaction to amplify viral DNA, successful gene transfer was achieved in 8 of 15 kidneys. In 6 of 10 kidneys assessed histochemically positive staining for beta-galactosidase activity was detected in the cytoplasm of tubular epithelial cells. There was no evidence of gene expression in glomerular, vascular or endothelial cells. All analyses were negative in vehicle-injected kidneys and in the kidneys of animals which did not receive pretreatment with folic acid. These studies demonstrate the feasibility of gene transfer into the adult kidney provided that replication of specific cell types can be achieved.