Sphingosine, a breakdown product of cellular sphingolipids, has recently been shown to stimulate DNA synthesis and act synergistically with known growth factors to induce proliferation of quiescent Swiss 3T3 fibroblasts (Hong, Z., Buckley, N. E., Gibson, K., and Spiegel, S. (1990) J. Biol. Chem. 265, 76-81). The present study demonstrates that mitogenic concentrations of sphingosine induce early increases in cytosolic phosphatidic acid, which is a potent mitogen for Swiss 3T3 cells. Structurally related analogs of sphingosine, such as N-stearoylsphingosine and other long chain aliphatic amines, did not mimic the mitogenic effect of sphingosine and did not elevate phosphatidic acid levels. Sphingosine not only stimulated [3H]thymidine incorporation with similar efficiency and kinetics as phosphatidic acid, it also induced similar morphological alterations. Both sphingosine and phosphatidic acid acted synergistically with a variety of growth factors, such as, insulin, epidermal growth factor, fibroblast growth factor, and 12-O-tetradecanoylphorbol 13-acetate. In sharp contrast, sphingosine and phosphatidic acid did not have additive or synergistic effects in either the presence or absence of other growth factors. Both sphingosine and phosphatidic acid stimulated DNA synthesis in cells made protein kinase C-deficient by prolonged treatment with phorbol ester and sphingosine still stimulated similar increases in phosphtidic acid in these cells. Furthermore, similar to the actions of phosphatidic acid on signal transduction in Swiss 3T3 cells, mitogenic concentrations of sphingosine also inhibit cAMP accumulation and trigger the hydrolysis of polyphosphoinositides. Our findings indicate that sphingosine and phosphatidic acid control cellular responses in Swiss 3T3 cells through a common pathway. In view of the prominent role of phosphatidic acid in signal transduction and cellular proliferation, our observations that sphingosine, at mitogenic concentrations, increases the level of phosphatidic acid and also mimics the effects of phosphatidic acid on signal transduction, have important implications for the mechanism of action of sphingosine.