Calcium signaling was observed in murine T cells over time, starting at a precise moment of contact with a layer of fibroblasts expressing a stimulatory major histocompatibility class II–peptide complex. The contact was controlled by a film-thinning apparatus. Intracellular calcium levels were followed with the ratiometric dye, Fura-2. The calcium response was highly synchronized and well fitted by a mathematical model. The model includes three components: a sequence of reactions occurring after T cell receptor (TCR) triggering; InsP₃-mediated calcium release from intracellular stores (Meyer and Stryer, Proc. Natl. Acad. Sci. USA 85: 5051–5055, 1988); and slow changes in levels phospholipase C-1 (PLC1) reflecting a decrease in receptor triggering rate. Each component in the model controls a different part of the response—the initial delay, the sharp rise, and the slow decay, respectively. Kinetic parameters determined from curve fitting were the initial delay in calcium signaling defined as the time when [PLC1] reached its half of its maximum (76 s), the coefficient characterizing calcium efflux from endoplasmic reticulum (ER) (2.86 M s^--1, expressed per liter of cell volume), and a rate constant characterizing the diminishing yield of production of PLC1 (0.00046 s^--1) by active TCR. Only the parameter representing PLC1 production varied much from cell to cell. © 2001 Biomedical Engineering Society.