To investigate the mechanism of tetanic stimulation induced enhancement of transmitter release, we analyzed two components of short-term synaptic plasticity, facilitation and potentiation, using binomial distribution with two parameters, release probability (p) and number of releasable synaptic vesicles (n). Frog neuromuscular junction was used as synapse preparation. Endplate potentials (EPPs) and miniature endplate potentials (MEPPs) were electrically recorded with an intracellular glass microelectrode in a low Ca²⁺ high Mg²⁺ Ringer's solution (0.50-0.75 mM Ca²⁺, 5 mM Mg²⁺). Facilitation was induced by 8 stimuli with interval of 25 or 30 msec. Potentiation was induced by 500 stimuli at 20 Hz. Then the preparation was kept 5 sec resting to ensure decay of facilitation and the remaining enhancement was maintained as steady state by 350 stimuli with an interval of 150-200 msec. The ratio of variance increase to mean increase of facilitated EPPs distribution was almost 1. However, that value of potentiated EPPs distribution was smaller than 1. These results suggests that the enhancement of transmitter release during facilitation and potentiation was caused mainly by the increase of n and p, respectively. The detailed binominal analyses of the change of two parameters, p and n, during facilitated and potentiated EPPs distributions confirmed that increase of n and p contributed largely to the enhancement of transmitter release in facilitation and potentiation, respectively. The effects of the existence of a series of distribution of p or n, not fixed to a constant value, were also examined with computer simulations. The Gaussian distribution of p produced almost the same result as p is fixed to its average value. The Gaussian distribution of n with S.D. ≥ 0.2×average produced the result as if n and p is fixed to a very large and small value, respectively.