Inoculation of adenovirus vectors in vivo has induced transgene expression in a variety of cochlear cells, but not hair cells or supporting cells in most previous studies. Specific hair cell inoculation by viral vectors has not been demonstrated in the mature guinea pig cochlea in vitro. We injected an Adex1CAlacZ into the mature guinea pig cochlear explants, which were incubated for 24-72 h. We found many lacZ-positive cells in a variety of tissues including the spiral ganglion and stria vascularis. Transgene expression was also found in the outer hair cells and supporting cells, such as the Deiters cells and pillar cells. These findings indicate that adenovirus vectors can be transfected into mature guinea pig hair cells and supporting cells in culture.