1. As a result of the cutting procedure there is a decrease in the membrane potential, input resistance, and space constant of the rat diaphragm, and there is no contraction when the phrenic nerve is stimulated. While the reversal potential of end-plate currents in cut preparations appears normal, the size of miniature end-plate currents (m.e.p.c.s.) is slightly decreased. 2. An increase in the external concentration of potassium from 2.5 to 10.0 mM results only in a minor change (less than 5%) in statistical parameters of transmitter release (m, n and p). The size of m.e.p.c. is also almost unchanged. 3. A decrease in temperature from 37 degrees to 15 degrees C resulted in a decrease of m, n and p; however, the values are less than 25% smaller at 25 degrees C than at 37 degrees C. The size of m.e.p.c.s is very insensitive to changes in temperature. 4. As previously reported for the frog neuromuscular junction, changes in muscle membrane potential of cut and uncut rat diaphragm due to voltage clamping affect the frequency of m.e.p.c.s only in medium with raised external K concentration. The dependence of frequency of m.e.p.c.s on muscle membrane potential is remarkably similar in cut and uncut preparations. Evoked release in cut preparation in normal medium is not affected by the change in muscle membrane potential.