With the aim of developing foetal gene therapy for cystic fibrosis, we have investigated the possibility of gene targeting to the mouse foetus with two different viral vector systems and at different times of gestation. We report here that recombinant retrovirus producing cells administered into the intra-amniotic cavity of mid- to late-gestation mouse MF1 foetuses survive in the amniotic fluid and are able to engraft to a certain extent in foetal tissues. By production of infectious virus they mediate transduction and beta-galactosidase transgene expression in neighbouring foetal tissues 24 to 72 h following injection. Retrovirus producer cells could, therefore, become a means to overcome the limitations of low retroviral titre, for in vivo foetal gene transfer. To investigate the developmental stage at which transduction of the airways and enteral systems can be obtained we also administered a highly infective first generation adenoviral vector (AdRSV beta gal) into the amniotic cavity of foetal mice between 13 to 16 days post coitus, beta-galactosidase activity was detected between 24 to 120 h after injection. The highest levels of transgene expression were generally observed between 48 to 72 h following injection of the adenoviral vector. We demonstrate that infection of the pulmonary airways is dependent on the developmental stage of the foetus and can be achieved on the 15th day of gestation.