Using quantitative anatomical techniques, we show that after intraorbital optic nerve transection in adult rats, virtually all retinal ganglion cells (RGCs) survive for 5 d and then die abruptly in large numbers, reducing the RGC population to approximately 50% of normal by day 7 and to less than 10% on day 14. During this period of rapid cell loss, some RGCs show cytochemical alterations indicative of apoptosis ("programmed cell death"), a change not previously categorized after axotomy in adult mammals. With intracranial lesions 8-9 mm from the eye, the onset of cell death is delayed until day 8 and is greater with cut than crush. The demonstration that axotomy results in apoptosis, the long interval between axonal injury and RGC death, and the different time of onset of the massive RGC loss with optic nerve lesions near or far from the eye suggest that axonal interruption triggers a cascade of molecular events whose outcome may be critically dependent on the availability of neuronal trophic support from endogenous or exogenous sources. The role of such molecules in RGC survival and the reversible nature of these injury-induced changes is underscored by the temporary rescue of most RGCs by a single intravitreal injection of brain-derived neurotrophic factor during the first 5 d after intraorbital optic nerve injury (Mansour-Robaey et al., 1994). The delayed pattern of RGC loss observed in the present experiments likely explains such a critical period for effective neurotrophin administration.