Generation of proapoptotic sphingolipids by neutral sphingomyelinase activation is an early response to hypoxia/reoxygenation (HR) in cardiomyocytes. Factor associated with neutral sphingomyelinase activation (FAN) mediates activation of sphingomyelinase and subsequent apoptosis. However, the participation of FAN in HR-induced cardiomyocyte cell death has not been elucidated. We therefore investigated the expression and role of FAN in rat cardiomyocytes. A cDNA was isolated from rat heart encoding putative rat FAN. Reverse transcriptase-polymerase chain reaction, immunoelectron microscopy, and immunofluorescence demonstrated for the first time the expression of FAN specifically in rat cardiomyocytes. FAN expression was confirmed by the finding that expression of a dominant-negative FAN almost completely abrogated HR-induced cell death, whereas overexpression of wild-type FAN led to an increase. Treatment of FAN and dominant-negative FAN–expressing cells with C2-ceramide produced substantial cell death, indicating dominant-negative FAN exerts its protective action by interfering with the activation of the sphingolipid cascade. Taking these results together, we conclude that FAN is a previously undescribed and important HR signaling component in the heart and that inhibition of FAN may provide a novel intervention point for reducing ischemia/reperfusion injury.