The effect of 4-aminopyridine (4-AP) on acetylcholine release was investigated on the rat phrenic diaphragmatic preparations by means of intracellular recording of spontaneous synaptic activity. 4-AP in concentrations of 1.10(-6) to 1910(-3) M did not cause significant shifts in the mean value of frequency and amplitude of miniature end-plate potentials (MEPP). At the same time 4-AP induced appearance of large spontaneous EPP capable of generating distribution of action potentials. 4-AP transformed the character of MEPP amplitude distribution into the polimodal one, the main node being shifted in several cases to the range of lower values. It was concluded that 4-AP can modify the character of acetylcholine release that intensifies the spontaneous synchronism manifestation.