Horseradish peroxidase (HRP) was injected into the retino-recipient nuclei of each hemisphere in newborn rats. The animals were perfused 3--9 days after the injections; the number of retinal ganglion cells in retinal wholemounts was estimated by counting cells containing granules of HRP reaction product. The mean number (150,500) of labelled cells in 3-day-old rats was significantly higher than those in older animals (117,000, 121,000, 113,000 respectively on 6th, 8th and 9th postnatal days). However, in animals of any of the ages studied, the estimated numbers of ganglion cells were virtually the same as those in the animals of the same age but injected with HRP only 15--20 h before the perfusion. Thus, the reduction in the number of retinal cells projecting to the central visual nuclei observed during the first few postnatal days is due to a wave of retinal ganglion cell death; ganglion cell death induced by the neonatal removal of the contralateral superior colliculus has a similar time course.