Cytoplasmic calcium concentration ([Ca],) rises within minutes of exposure of T lymphocytes to a mitogen. T cells from old mice are defective in this reaction, a defect that could reflect either altered signal transduction or instead a more general age-associated change in intracellular calcium regulation. We therefore tested the ability of T cells from old mice to regulate their [Ca], concentration after exposure to low concentrations of ionomycin, an agent that raises [Ca], but bypasses receptor-mediated signal transduction mechanisms. Exposure of T cells to ionomycin leads to an abrupt increase in [Ca], followed by stabilization at a dose-dependent plateau level that is affected by extracellular EGTA, by calmodulin inhibitors, and by modulators of protein kinase C. Plateau levels of [Cal, after ionomycin challenge were consistently lower in T cells from old mice than in T cells from young mice. Flow cytometric experiments showed that while essentially all T cells from both old and young mice responded to ionomycin, they did so to an extent that depended on donor age. The age-dependent increase in resistance to ionomycin-induced changes in [Ca], cannot be attributed to diminished membrane permeability to the ionomycin-calcium complex. The data suggest that aging may lead, in T lymphocytes, to a relative resistance to increases in [Ca],, a resistance that in turn prevents cell activation.