Purpose. To determine whether a reporter gene can be introduced into the adult mammalian retina in vivo through means of a recombinant replication- deficient adenovirus. Methods. A dilution series of purified Ad.CMVlacZ ranging from 105 to 1011 pfu/ml was prepared and microinjected into the subretinal space of adult CD-1 mice. This virus contained the cytomegalovirus (CMV)-promoted Escherichia coli reporter gene, lacZ. LacZ expression was assessed in enucleated eyes from 0 to 95 days after injection by β- galactosidase (β-Gal) assay. Results. The efficiency of transfection increased as a function of concentration of recombinant virus injected. Eyes injected with greater than 107 pfu of Ad.CMVlacZ demonstrated β-Gal activity lasting at least 95 days. LacZ expression was apparent only in those cells directly exposed to the adenovirus. LacZ expression was observed in the retinal pigment epithelium (RPE) at high efficiency at 48 hours after exposure. By 2 weeks after injection of >107 pfu, lacZ was also expressed in photoreceptors but at lower density. Conclusions. These results demonstrate that high efficiency stable transfer of functional genes can be achieved in vivo in post-mitotic mammalian retina using recombinant adenoviral vectors. Adenovirus vectors appear to be a promising means for delivering therapeutic genes in vivo to the mammalian neural retina and particularly to the RPE.