Tang et al.(2000) demonstrated that, at the crayfish neuromuscular junction, both the accumulation and the decay properties of short-term synaptic facilitation (STF) are strongly affected by the addition of a fast high-affinity Ca2 buffer, suggesting a role of residual free Ca2 in the induction of STF. The authors proposed that the experimental results can be explained by a secretion model with two Ca2 binding sites, a secretory site mediating exocytosis and located close to the Ca2 channel (10–20 nm), and a high-affinity facilitation site located further away (80–100 nm) from the channel. Here we report that the data presented in Figs. 3, C and D, 6, and 7 of the original article, showing numerical solutions to the Ca2 diffusion equations, are qualitatively inaccurate, because of misstated parameter values and, to a lesser extent, numerical algorithm errors. Therefore, some of the conclusions stated by Tang et al. concerning the proposed model require reexamination. In this letter we show that most of the predictions of the model hold, after an appropriate change of parameter values. Namely, the model correctly predicts the magnitude of STF, and the reduction of STF magnitude and acceleration of its decay in the presence of a fast high-affinity exogenous Ca2 buffer, such as Fura-2. The fast (“F1”) and slow (“F2”) decay components of STF are also successfully reproduced, if an additional assumption is made that the endogenous Ca2 buffers are immobile. However, our simulations predict that the slower F2 decay component is completely abolished in the presence of Fura-2, contrary to experimental results of Tang et al.(2000)(Fig. 3, A and B, in the original paper). We found that this remaining disagreement can be resolved if one assumes that the diffusion in the synaptic bouton is restricted, so that 1) in a 200-nm layer around the active zone, Fura-2 is immobilized and the diffusion coefficient of Ca2 is reduced fivefold, due to a high degree of tortuosity; and 2) in the rest of the terminal, the diffusion coefficient of Fura-2 is reduced 100-fold (presumably because of binding to various cytosolic compounds). Moreover, contrary to the statement by Tang et al. that their model fails to accurately describe the accumulation time course of STF, we show that the model modifications that we propose lead to a supralinear growth of STF, in agreement with experiment (Fig. 2, A and D).