There is growing evidence that caspase inhibition exerts neuroprotective effects in various models of neuronal injury in vivo. However, whether caspase inhibition provides long-term neuroprotection is not known yet. In the present study, we therefore investigated the effects of prolonged caspase inhibition on the survival of adult rat retinal ganglion cells (RGCs) following optic nerve (ON) transection. Four weeks following ON transection the number of surviving RGCs in untreated animals declined to 11% of controls. Treatment for the initial 2 weeks with z-DEVD-cmk, an irreversible inhibitor of ced 3-like caspases, increased the number of surviving RGCs 4 weeks postlesion to 24%. Z-DEVD-cmk treatment over the entire experimental period of 4 weeks had no additional effect. Thus, we still found a neuroprotective effect of caspase inhibition on axotomized RGCs after extended survival time. However, in comparison to our recent observations 2 weeks after optic nerve transection, in which z-DEVD-cmk rescued 46% of RGCs (P. Kermer, N. Klöcker, M. Labes, and M. Bähr, 1998, J. Neurosci. 18(12), 4656-4662) the positive effect clearly decreased. In conclusion, our results indicate that the therapeutical approach presented here results in a significant delay of secondary death rather than providing a permanent and complete rescue of axotomized RGCs.