Some molluscan ganglion cells were hyperpolarized by an excess of external K, contrary to expectations by the Nernst equation. The membrane resistance of the cells was markedly decreased with the hyperpolarization. This phenomenon was considered to represent a result of the summated IPSP's elicited by the presynaptic inhibitory fibers which were primarily depolarized by high K. We examined the effect of Cd in low pH saline on the membrane potential of snail ganglion cells in order to analyze this phenomenon further. Since Cd has been reported to block Ca channels, it might prevent neurotransmitter release from the presynaptic ending. In pH 8 control saline, the excess of K (22 mM) caused significant hyperpolarization and an evident decrease in membrane resistance, and these changes were not modified much after 1mM Cd addition. In pH 6.5 and 5 saline, the high K-induced hyperpolarization was markedly suppressed or even changed to depolarization by 1 mM Cd. The resistance change was also decreased in low pH Cd saline. Cadmium exerted a greater effect at rather higher concentrations. The results obtained suggest that the high K-induced hyperpolarization is due to the release of inhibitory neurotransmitter.