Ionizing radiation induces apoptosis in bovine aortic endothelial cells (BAEC) by activating the sphingomyelin pathway, leading to ceramide production. This study shows that radiation, like tumor necrosis factor (TNF)-α, rapidly hydrolyzes sphingomyelin to ceramide, which then activates a ceramide-activated serine/threonine protein kinase, initiating apoptosis. Exogenous ceramide analogues can mimic this effect, while protein kinase C (PKC) activation blocks both ceramide generation and apoptosis. These findings suggest that radiation-induced apoptosis is mediated through membrane signals rather than direct DNA damage. The sphingomyelin pathway is a key signaling mechanism for apoptosis, involving sphingomyelinase activity that generates ceramide, which acts as a second messenger to trigger apoptosis. Radiation-induced ceramide generation occurs in membrane preparations devoid of nuclei, indicating that membrane signaling is sufficient for apoptosis without involving the nucleus. The study provides evidence that ionizing radiation can generate apoptotic signals through interaction with cellular membranes, offering an alternative to the hypothesis that direct DNA damage is the primary mechanism of radiation-induced cell death. The results highlight the role of ceramide in radiation-induced apoptosis and suggest that membrane signaling pathways, including sphingomyelin metabolism, are critical in the apoptotic response to ionizing radiation.Ionizing radiation induces apoptosis in bovine aortic endothelial cells (BAEC) by activating the sphingomyelin pathway, leading to ceramide production. This study shows that radiation, like tumor necrosis factor (TNF)-α, rapidly hydrolyzes sphingomyelin to ceramide, which then activates a ceramide-activated serine/threonine protein kinase, initiating apoptosis. Exogenous ceramide analogues can mimic this effect, while protein kinase C (PKC) activation blocks both ceramide generation and apoptosis. These findings suggest that radiation-induced apoptosis is mediated through membrane signals rather than direct DNA damage. The sphingomyelin pathway is a key signaling mechanism for apoptosis, involving sphingomyelinase activity that generates ceramide, which acts as a second messenger to trigger apoptosis. Radiation-induced ceramide generation occurs in membrane preparations devoid of nuclei, indicating that membrane signaling is sufficient for apoptosis without involving the nucleus. The study provides evidence that ionizing radiation can generate apoptotic signals through interaction with cellular membranes, offering an alternative to the hypothesis that direct DNA damage is the primary mechanism of radiation-induced cell death. The results highlight the role of ceramide in radiation-induced apoptosis and suggest that membrane signaling pathways, including sphingomyelin metabolism, are critical in the apoptotic response to ionizing radiation.