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1. Depolarizing currents were applied to motor nerve terminals in the rat phrenic nerve—diaphragm muscle preparation in vitro.

2. During the passage of depolarizing currents the amplitude of the presynaptic nerve action potentials and of end-plate potentials (e.p.p.s) was reduced in proportion to the current strength.

3. The reduction in e.p.p. amplitudes was shown to be due to a reduction in the number of quanta released.

4. An excess of Mg ions or the previous application of a hyperpolarizing current could prevent the reduction of e.p.p. amplitudes and quantal contents by depolarizing current.

5. Depolarizing current application prevented later hyperpolarizing currents affecting e.p.p. amplitudes.