The purpose of this study was to investigate how muscle stretching affects the activity of neuromuscular transmission. The magnitude of the post-tetanic potentiation (PTP) of miniature end-plate potential (m.e.p.p.) frequency was measured in the rat soleus muscle at resting length and stretched length. The parameters of the magnitude of PTP can be indicators of the kinetics of Ca2+ metabolism in the nerve terminal. One of the parameters, normalized initial post-tetanic frequency (f), was significantly increased by stretching muscle 10% (P less than 0.05) and 20% (P less than 0.01) of its resting length. Another parameter, the time constant of augmentation (tau a), was not significantly changed by muscle stretching. The time constant of potentiation (tau p) was significantly increased by 20% muscle stretching (P less than 0.05). These results indicate that the Ca2+ conductance, especially the voltage-dependent Ca2+ influx, of the nerve terminal could be increased by muscle stretching. Greater Ca2+ conductance of the nerve terminal would increase intracellular free Ca2+. Consequently, the probability of transmitter release would be increased, because Ca2+ is closely related to the activity of the transmitter release mechanism.