1. The nerve-muscle preparation of the cutaneous pectoris of the frog has been used to study quantal transmitter release.2. When the osmotic pressure of the external solution is raised 1.5-2 fold, the frequency of miniature end-plate potentials (m.e.p.p.s) rises by 1.5-2 orders of magnitude. This effect is independent of the presence of Ca(2+) ions and of the nature of the substances by which the osmotic pressure has been increased.3. In Ca(2+) free hypertonic solution the nerve impulse still invades the nerve terminals but does not alter the frequency of the m.e.p.p.s.4. The arrival of the impulse in the terminals causes an immediate increase in the rate of quantal release, provided divalent cations are present whose passage through the axon membrane is facilitated by excitation (Ca(2+), Sr(2+), Ba(2+)).5. Divalent cations which penetrate only slightly (Mg(2+), Be(2+)) lower the frequency of m.e.p.p.s and suppress the end-plate potential (e.p.p.) evoked by an impulse, in the presence of Ca(2+) ions. Be(2+) is a more effective inhibitor than Mg(2+).6. In Ca(2+) free solutions, adding Mg(2+) causes an increase in the frequency of m.e.p.p.s evoked by depolarization of the nerve endings or by treatment with ethanol.7. The trivalent cation La(3+) is more effective than divalent cations are in increasing the frequency of m.e.p.p.s. The tetravalent cation Th(4+) also raises the m.e.p.p. frequency.8. The observations summarized in paragraphs 2-7 indicate that the frequency of m.e.p.p.s at a constant temperature depends only on the concentration of uni-, di- and trivalent cations inside the nerve ending. It is suggested that the internal cation concentration influences the adhesion between synaptic vesicles and the membrane of the nerve ending.9. For a model experiment, artificial phospholipid membranes have been used to study the effect of uni-, di-, tri- and tetravalent cations on the adhesion process. At pH 7-7.4, the time required for adhesion to take place decreases with increasing cation concentration in the bath. Ca(2+) ions are 100-1000 times more effective than K(+) ions; La(3+) and Th(4+) ions are still more effective. The ;adhesion time' decreases when the pH is lowered; it increases greatly with lowering of temperature.10. The hypothesis is put forward that the mutual adhesion of artificial vesicles made of phospholipid membranes, and the adhesion between synaptic vesicles and the membrane of the nerve ending arise by a common mechanism. In both cases, the important factor is the influence of cations on the electric double layer at the membrane surface.