In the developing central nervous system (CNS), the differentiation of neurons is accompanied by a large amount of cell loss in the form of programmed cell death (apoptosis). On the other hand, injury in the adult CNS often results in permanent neuronal degeneration leading to the failure of axonal regeneration. This could be related to an axotomy-induced activation of an apoptotic program. The interleukin-1beta-converting enzyme family of proteases has been implicated in playing a fundamental role in apoptotic processes in both invertebrates and vertebrates. In order to determine what role, if any, inhibitors of the interleukin-1beta-converting enzyme family might play in axonal regeneration after axotomy we employed an in vitro system using retinal explants from post-natal rats at the age between 9 and 12 days [R. Lucius, P. Young, S. Tidow and J. Sievers, Growth stimulation and chemotropic attraction of retinal ganglion cell axons in vitro by co-cultured optic nerves, astrocytes and astrocyte conditioned medium, Int. J. Dev. Neurosci., Vol. 14 (1996) 387-398]. The retinal ganglion cells in this model are comparable to adult animals in their regenerative response (D. Alcutt, M. Berry and J. Sievers, A qualitative comparison of the reaction of retinal ganglion cells to optic nerve crush in neonatal and adult mice, Dev. Brain Res., Vol. 16 (1984a) 231-240; D. Allcutt, M. Berry and J. Sievers, A quantitative comparison of the reaction of retinal ganglion cells to optic nerve crush in neonatal and adult mice, Dev. Brain Res., Vol. 16 (1984b) 219-230]. The addition of the synthetic peptide inhibitor YVAD (Ac-Tyr-Val-Ala-Asp-aldehyde), which reversible inhibits interleukin-1beta-converting enzyme and subsequent apoptosis, enhances the number and length of regenerating neurites of retinal ganglion cells. However, this manipulation was not able to overcome free radical-induced axonal degeneration/neuronal apoptosis, suggesting at least two different death pathways leading to neuronal degeneration/cell death. These results provide the first evidence that inhibitors of key enzymes of the apoptotic program could play a beneficial role to overcome neurite degeneration after axotomy in the adult mammalian CNS.