To determine whether Ca2+ participates in the augmentative action of SnCl2 on the transmitter release, we have investigated interactions between SnCl2 and Cd2+, verapamil, or low Ca2+ on endplate potentials (e.p.p.) and have compared the effect of SnCl2 with that of Mg2+ on miniature endplate potentials (m.e.p.p.). SnCl2 increased the e.p.p. amplitude dose-dependently. Cd2+, verapamil and low Ca2+ shifted the dose-response relationship between SnCl2 and the e.p.p. downwards. In contrast to Mg2+, SnCl2 increased the m.e.p.p. frequency with raised K+ concentration in the medium. These results suggest that Ca2+ participates in the SnCl2-induced facilitation of the transmitter release and that SnCl2 enhances the Ca2+ entry into the nerve terminals.