The effects of neonatal midbrain lesions on populations of retinal ganglion cells with ipsilateral or contralateral projections were investigated in hooded rats with the use of horseradish peroxidase. After bilateral lesions of the superior colliculus performed at birth, the number of contralaterally projecting ganglion cells is reduced but the number of ipsilaterally projecting cells is increased. Bilateral tectal lesions performed 5 days after birth reduce the number of both contralaterally and ipsilaterally projecting ganglion cells. Unilateral tecto-pretectal lesions performed at birth lead to extensive retrograde degeneration of contralaterally projecting ganglion cells in the opposite retina: but both the ipsilateral terminal fields of the same retina and its population of ipsilaterally projecting ganglion cells are increased. Cells located at the border between the temporal crescent and the nasal areas of the retina opposite an unilateral tecto-pretectal lesion were found to have their dendrites pointing towards the severely depleted nasal areas more frequently than in normal rats. These observations suggest that competitive interactions between retinal dendrites may play a role in regulating ganglion cell death in the developing retina. The increased population of ipsilaterally projecting ganglion cells would reflect the survival of neurones which would otherwise normally degenerate, resulting from reduced local interactions as a consequence of the massive removal of neighbouring contralaterally projecting cells.