Radiation leads to a rapid burst of reactive oxygen species (ROS), which is considered to be one of the major causes of radiation-induced injury. ROS have previously been shown to induce changes in cytosolic Ca² ([Ca² ]_i) including [Ca² ]_i oscillation. However, the role of radiation in [Ca² ]_i oscillation is poorly understood. The purpose of this study was to identify the effect of ROS and X ray on [Ca² ]_i oscillation, as well as their role in radiation-induced lung injury. Alveolar macrophages were cultured in the absence and presence of different doses of hydrogen peroxide (H₂O₂) or exposed to X-ray irradiation with or without pretreatment of diphenyleneiodonium chloride (DPI, an inhibitor of NADPH oxidases) or tetrandrine (TET, a calcium entry blocker) and cytosolic Ca² concentration was detected by fluorescent Ca² indicator Fura-2. Rat radiation lung injury was induced in vivo by using 40 Gy X ray and DPI or TET was used to prevent radiation-induced lung injury. The results showed that there was spontaneous [Ca² ]_i oscillation in alveolar macrophages under normal conditions, and treatment of H₂O₂ (100–500 μM) or 2 Gy X ray inhibited the spontaneous [Ca² ]_i oscillation and induced [Ca² ]_i rise. TET abolished H₂O₂ or X ray induced [Ca² ]_i rise in alveolar macrophages, and attenuated X ray- induced rat alveolitis in vivo. DPI prevented X-ray-induced inhibition of [Ca² ]_i oscillation in alveolar macrophages and prevented X-ray-induced rat alveolitis. Taken together, the data suggest that the disruption of [Ca² ]_i oscillation and induction of [Ca² ]_i rise through ROS is involved in the mechanism of radiation-induced lung injury.