Propylene glycol (PG) has an excitatory effect on the skeletal muscle of the frog. To determine whether PG has a facilitating effect on the neuromuscular transmission of the mammalian as well as on that of the amphibian and to elucidate the mode of action, we have investigated the effects of PG on the neuromuscular junction of the mouse. PG (1.0% v/v) significantly increased the amplitude of endplate potential. PG raised the frequency of miniature endplate potential and increased its amplitude. PG increased the mean quantal content of the endplate potential. These results indicate that PG facilitates the mouse neuromuscular transmission by accelerating the transmitter release from the nerve terminals and by raising the acetylcholine sensitivity of the endplates.