CERAMIDE is an important regulatory participant of programmed cell death (apoptosis) induced by tumour-necrosis factor (TNF)-alpha and Fas ligand, members of the TNF superfamily(1-6). Conversely, sphingosine and sphingosine-1-phosphate, which are metabolites of ceramide, induce mitogenesis(7) and have been implicated as second messengers in cellular proliferation induced by platelet-derived growth factor and serum(8,9). Here we report that sphingosine-1-phosphate prevents the appearance of the key features of apoptosis, namely intranucleosomal DNA fragmentation and morphological changes, which result from increased concentrations of ceramide. Furthermore, inhibition of ceramide-mediated apoptosis by activation of protein kinase C results from stimulation of sphingosine kinase and the concomitant increase in intracellular sphingosine-l-phosphate. Finally sphingosine-1-phosphate not only stimulates the extracellular signal-regulated kinase (ERK) pathway(10), it counteracts the ceramide-induced activation of stress-activated protein kinase (SAPK/JNK). Thus, the balance between the intracellular levels of ceramide and sphingosine-1-phosphate and their regulatory effects on different family members of mitogen-activated protein kinases determines the fate of the cell.