Transmitter quanta were elicited from nerve terminals of crayfish and frog muscle by depolarization pulses through a macro-patch-clamp electrode. The rates of quantal release for twin pulses and their ratio, twin pulse facilitation, Fd, were determined. When the electrode was perfused with normal Cae (13.5 mM for crayfish, 1.8 mM for frog), Fd was low for threshold depolarizations, increased to a maximum at medium depolarizations, decreased when the rate of release due to the first pulse approached saturation, and increased again for larger depolarizations. If under these conditions the superfusion of the muscle outside the electrode was changed from normal to 0 Cab and high Mgb solution, Fd increased. When the Ca concentration around the terminal, Cae, was reduced to levels at which release did not reach the saturation level for large depolarizations, Fd in dependence on depolarization did not show the minimum at higher depolarizations. The amplitude of Fd measured for large, constant depolarization pulses showed a maximum at a Cae below that of the normal solution. The maximum of Fd was much higher if the superfusion of the bath contained 0 Cab and high Mgb than when normal bathing solution was superfused. The maxima of Fd at a low value of Ca inflow are predicted by the "residual Ca" theory of facilitation, if release is influenced by a resting low internal Ca concentration, Cair, and reaches a saturation level for large Ca-inflow. It is also predicted that decreasing Cair (as in low Cab) will increase Fd.