T cell receptor (TCR) stimulation, leading to T cell activation and ultimately to cell proliferation and differentiation, evokes elevations of [Ca2+]i with a high variability between individual T lymphocytes. We have used Ca(2+)-imaging of Fura-2 loaded cells to study the origin of the variation in Ca2+ signals and its consequences for the final cellular response. We found that, compared to resting cells, the percentage of responding cells and the average amplitude of the Ca2+ signal upon TCR re-stimulation by PHA increases in the first 5 days of T cell activation and declines thereafter, with more pronounced [Ca2+]i oscillations in later stages. In parallel, an enhancement of T cell proliferation is observed. Stronger stimulation of the TCR/CD3 complex by co-crosslinking CD3 with CD4/CD8 molecules evokes oscillating Ca2+ responses irrespective of the activation state, indicating that the basic capacity for Ca2+ signaling is essentially the same in resting and activated cells. Nevertheless, also the amplitude of the CD3+CD4/8 response shows a transient additional increase during the first days of T cell activation. Experiments with the K+ channel blocker charybdotoxin (CTX) indicate that [Ca2+]i oscillations depend critically on K+ channel functioning, but suppression of these oscillations by CTX does not significantly affect the average amplitude of the Ca2+ signal nor PHA-induced proliferation. However, when applied during the first 4-5 days of activation, CTX reduces in addition the average level of the TCR evoked Ca2+ response and inhibits subsequent proliferation.