GROWTH signalling networks that use glycerophospholipid metabolites as second messengers have been well characterized^1,2, but less is known of the second messengers derived from sphingolipids^3,4, another major class of membrane lipids. A tantalizing link between sphingolipids and cellular proliferation has emerged from the discovery that the sphingolipid metabolites sphingosine and sphingosine-1-phosphate stimulate growth of quiescent Swiss 3T3 fibroblasts by a pathway that is independent of protein kinase C^5,6. Sphingosine-1-phosphate is rapidly produced from sphingosine and may mediate its biological effects⁶. Furthermore, sphingosine-1-phosphate triggers the dual signal transduction pathways of calcium mobilization^6,7 and activation of phospholipase D⁸, prominent events in the control of cellular proliferation. Here we report that activation of sphingosine kinase and the formation of sphingosine-1-phosphate are important in the signal transduction pathways activated by the potent mitogens platelet-derived growth factor (PDGF) and fetal calf serum (FCS).