Calcium dynamics associated with a single action potential were studied quantitatively in the calyx of Held, a large presynaptic terminal in the rat brainstem. Terminals were loaded with different concentrations of high- or low-affinity Ca2+ indicators via patch pipettes. Spatially averaged Ca2+ signals were measured fluorometrically and analyzed on the basis of a single compartment model. A single action potential led to a total Ca2+ influx of 0.8-1 pC. The accessible volume of the terminal was about 0.4 pl; thus the total calcium concentration increased by 10-13 microM. The Ca(2+)-binding ratio of the endogenous buffer was about 40, as estimated from the competition with Fura-2, indicating that 2.5% of the total calcium remained free. This is consistent with the peak increase in free calcium concentration of about 400 nM, which was measured directly with MagFura-2. The decay of the [Ca2+]i transients was fast, with time constants of 100 ms at 23 degrees C and 45 ms at 35 degrees C, indicating Ca2+ extrusion rates of 400 and 900 s-1, respectively. The combination of the relatively low endogenous Ca(2+)-binding ratio and the high rate of Ca2+ extrusion provides an efficient mechanism for rapidly removing the large Ca2+ load of the terminal evoked by an action potential.