Repetitive stimulation of motor nerves causes an increase in the number of packets of transmitter ("quanta") that can be released in the ensuing period. This represents a type of conditioning, in which synaptic transmission may be enhanced by prior activity. Despite many studies of this phenomenon, there have been no investigations of the quantal mechanisms underlying these events, due to the rapid changes in transmitter output and the short time periods involved. To examine this problem, a method was developed in which estimates of the quantal release parameters could be obtained over very brief periods (3 s). Conventional microelectrode techniques were used to record miniature endplate potentials (MEPPs) from isolated frog (Rana pipiens) cutaneous pectoris muscles, before and after repetitive (40 sec at 80 Hz) nerve stimulation. Estimates were obtained of m (number of quanta released), n (number of functional release sites), p (mean probability of release) and var$\rm\sb{s}$p (spatial variance in p) using a method that employs counts of MEPPs per unit time. Fluctuations in the estimates were reduced using a moving bin technique (bin size = 3 s, $\Delta$bin = 1 s). Muscle contraction was prevented using low Ca$\sp{2+},$ high Mg$\sp{2+}$ Ringer or normal Ringer to which $\mu$-conotoxin GIIIA was added. These studies showed that: (1) the post-stimulation increase in transmitter release was dependent on stimulation frequency and not on the total number of stimulus impulses. When the total number of pulses was kept constant, the high frequency pattern produced a higher level of transmitter release than did the lower frequency patterns; (2) augmentation and potentiation were present in both low Ca$\sp{2+},$ high Mg$\sp{2+}$ and normal Ringer solutions, but potentiation, m, n, p and var$\rm\sb{s}$p were greater in normal Ringer solution than in low Ca$\sp{2+},$ high Mg$\sp{2+}$ solution. In low Ca$\sp{2+},$ high Mg$\sp{2+}$ solution, there was a larger decrease in n compared to p; (3) hypertonicity (addition of 100 mM sucrose) produced a marked increase in both basal and stimulation-induced values of m, n, and p. By contrast, there was a marked increase in the stimulation-induced but not the basal values of var$\rm\sb{s}$p; (4) hypertonicity produced a decrease in augmentation but had no effect on potentiation; (5) augmentation and potentiation appeared to involve mitochondrial uptake and efflux of cytoplasmic Ca$\sp{2+}.$ Tetraphenylphosphonium (which blocks mitochondrial Ca$\sp{2+}$ efflux and uptake) decreased augmentation and potentiation in low Ca$\sp{2+},$ high Mg$\sp{2+}$ solutions but increased potentiation in the same solution made hypertonic with 100 mM sucrose; (6) the overall findings suggest that this new method may be useful for investigating the subcellular dynamics of transmitter release following nerve stimulation.