Although a mixture composed of 30 microM stannous chloride (SnCl2) and 75 microM tartaric acid (TA) facilitates neuromuscular transmission in the frog, the mixture had no effect on the endplate potential (e.p.p.) in the rat. The present study has been undertaken to determine whether the responses of the mouse (mammalian) to SnCl2 are different from those of the frog (amphibian). The mixture had no effect on the resting potential or the membrane resistance of the muscle fiber. The mixture did not change the e.p.p. amplitude, but TA (75 microM) significantly decreased it. The mixture increased the quantal content of the e.p.p., but TA decreased it. The mixture raised the frequency of the miniature endplate potential (m.e.p.p.) in the high potassium-medium. In normal saline, both the mixture and TA decreased both the m.e.p.p. frequency and its amplitude. These results suggest that SnCl2 itself facilitates the transmitter release from nerve terminals in the mammalian as well as in the amphibian species.