Singlet oxygen (¹O₂)–induced cytotoxicity is believed to be responsible for responses to photodynamic therapy and for apoptosis of T helper cells after UV-A treatment. Other cytotoxic oxidants, such as hydrogen peroxide and peroxynitrite have been shown to stimulate cell survival signaling pathways in addition to causing cell death. Both these oxidants stimulate the Akt/protein kinase B survival signaling pathway through activation of membrane tyrosine kinase growth factor receptors. We evaluated the ability of ¹O₂ to activate the Akt/protein kinase B pathway in NIH 3T3 cells and examined potential activation pathways. Exposure of fibroblasts to ¹O₂ elicited a strong and sustained phosphorylation of Akt, which occurred concurrently with phosphorylation of p38 kinase, a proapoptotic signal. Inhibition of phosphatidylinositol-3-OH kinase (PI3-K) completely blocked Akt phosphorylation. Significantly, cell death induced by ¹O₂ was enhanced by inhibition of PI3-K, suggesting that activation of Akt by ¹O₂ may contribute to fibroblast survival under this form of oxidative stress. ¹O₂ treatment did not induce phosphorylation of platelet-derived growth factor receptor (PDGFR) or activate SH-PTP2, a substrate of growth factor receptors, suggesting that PDGFR was not activated. In addition, specific inhibition of PDGFR did not affect Akt phosphorylation elicited by ¹O₂. Activation of neither focal adhesion kinase (FAK) nor Ras protein, both of which mediate responses to reactive oxygen species, appeared to be pathways for the ¹O₂-induced activation of the PI3-K–Akt survival pathway. Thus, activation of Akt by ¹O₂ is mediated by PI3-K and contributes to a survival response that counteracts cell death after ¹O₂-induced injury. However, unlike the response to other oxidants, activation of the PI3-K–Akt by ¹O₂ does not involve activation of growth factor receptors, FAK or Ras protein.