Horseradish peroxidase (HRP) was injected into both sides of the brain of newborn and adult rats. The number of retinal ganglion cells was estimated by counting cells containing granules of HRP reaction product. The mean number of labelled cells in 2-day-old animals was 169,500 (S.D. +/- 16,000, n = 6). By the tenth postnatal day the mean number of labelled cells had fallen to 113,500 (+/- 2900, n = 3). This value is similar to the mean number of labelled cells in the adult animal (113,000 +/- 2700; n = 4). Thus, during the first few postnatal days the number of retinal cells projecting to the central visual nuclei is reduced by at least 35%.