The novel lipid second messenger, ceramide, induced apoptosis in PC12 cells as determined morphologically by nuclear appearance and internucleosomal DNA fragmentation, Apoptosis was induced by exogenous C-2-ceramide in a dose- and time-dependent manner, Natural ceramide and C-6-ceramide had a similar effect, This response mas specific since the structural analog C-2-dihydroceramide and other related lipids failed to initiate apoptosis, The apoptotic effect of ceramide also depends critically on cell plating density, Furthermore, the peptide inhibitor of interleukin-1 beta converting enzyme (ICE)-like proteases, Z-VAD.FMK, completely prevented the nuclear changes induced by ceramide, implicating the involvement of ICE-like protease activation in ceramide-induced apoptosis in PC12 cells.