Previous methods of in vivo gene transfer to differentiated neurons of the adult mammalian brain have been inefficient and associated with technical problems. We have therefore developed a model system of direct gene transfer using a replication–defective adenoviral vector containing a β–galactosidase gene to transduce brain neurons. Following injection of purified high titre recombinant adenovirus into the caudate putamen of seven week old mice, lacZ activity was evident in neural components of the central nervous system (CNS) for at least 8 weeks post infection. The efficiency of adenoviral gene transfer was very high compared to other techniques, suggesting an attractive and efficient alternative for neuronal gene transfer in vivo.