T cells play a central role in acquired immunity by recognizing foreign antigens through T-cell antigen receptor (TCR). Upon antigen recognition, two major downstream signaling pathways are activated. One is the calcium/calcineurin pathway and the other is the Ras-ERK MAPK cascade. T-cell repertoire is established in the thymus by TCR-dependent positive and negative selection. Negative selection of autoreactive thymocytes occurs in the developing CD4+CD8+ stage and is mediated by calcium-dependent apoptotic cell death. Developing CD4+CD8+ thymocytes are known to be highly susceptible to apoptotic cell death. We reported that the activation of the calcium/calcineurin pathway is required for positive selection but not negative selection. We established a video-imaging system that is able to analyze the intracellular calcium ion concentration ([Ca2+]i) induced by cell-cell interaction. We found that the increased [Ca2+]i in thymocytes was sustained for more than 5 min, while in mature T cells in the periphery it decreased significantly within a minute. These results suggest that calcium mobilization induced by TCR recognition of antigen in T cells is developmentally regulated. These qualitatively distinct calcium responses may reflect a difference in susceptibility to calcium-dependent apoptotic cell death between thymocytes and mature T cells.