The deregulation of E2F activity is thought to contribute to the uncontrolled proliferation of many tumor cells. While the effects of overexpressing E2F genes have been studied extensively in tissue culture, the consequences of elevating E2F activity in vivo are unknown. To address this issue, transgenic lines of Drosophila were studied in which ectopic expression of dE2F and dDP was targeted to the developing eye. The co-expression of dDP or dE2F disrupted normal eye development, resulting in abnormal patterns of bristles, cone cells and photoreceptors. dE2F/dDP expression caused ectopic S phases in post-mitotic cells of the eye imaginal disc but did not disrupt the onset of neuronal differentiation. Most S phases were seen in uncommitted cells, although some cells that had initiated photo-receptor differentiation were also driven into the cell cycle. Elevated expression of dE2F and dDP caused apoptosis in the eye disc. The co-expression of baculovirus p35 protein, an inhibitor of cell death, strongly enhanced the dE2F/dDP-dependent phenotype. These results show that, in this in vivo system, the elevation of E2F activity caused post-mitotic cells to enter the cell cycle. However, these cells failed to proliferate unless rescued from apoptosis.