Abstract Exposure of lymphoid cells to X-irradiation caused an increase in cytosolic Ca 2+ ([Ca 2+] 1) in rat thymocytes but not in rat splenocytes. The Ca 2+ elevation in rat thymocytes was transitory and by 2 h postirradiation, when the chromatin degradation began to be appreciable, the [Ca 2+] i had returned to control level. Inhibitors of RNA and protein syntheses prevented the radiation-induced [Ca 2+] 1 rise in thymocytes. Pretreatment of both cell types with Ca 2+ chelators also prevented DNA fragmentation, chromatin condensation, and cell death caused by X-irradiation. These data suggest that Ca 2+ plays an important role in the perpetuation of apoptosis in both thymocytes and splenocytes although a Ca 2+ elevation, which may serve as a signal in thymocytes, does not appear to be required to initiate radiation-induced DNA fragmentation in splenocytes.