Radiation therapy for variety of human solid tumors utilizes mechanism of cell death after DNA damage caused by radiation. In response to DNA damage, cytochrome c was released from mitochondria by activation of pro-apoptotic Bcl-2 family proteins, and then elicits massive Ca2＋ release from the ER that lead to cell death. It was also suggested that irradiation may cause the deregulation of Ca2＋ homeostasis and trigger programmed cell death and regulate death specific enzymes. Thus, in this study, we investigated how cellular Ca2＋ metabolism in RKO cells, in comparison to radiation-resistant A549 cells, was altered by gamma (γ)-irradiation. In irradiated RKO cells, Ca2＋ influx via activation of NCX reverse mode was enhanced and a decline of [Ca2＋]i via forward mode was accelerated. The amount of Ca2＋ released from the ER in RKO cells by the activation of IP3 receptor was also enhanced by irradiation. An increase in [Ca2＋]i via SOCI was enhanced in irradiated RKO cells, while that in A549 cells was depressed. These results suggest that γ-irradiation elicits enhancement of cellular Ca2＋ metabolism in radiation-sensitive RKO cells yielding programmed cell death.