Divalent cations and temperature dependent block of impulse propagation at the frog neuromuscular junction

End-plate potentials were recorded from superficial muscle fibers of the frog sartorius nerve-muscle preparation. Exposure of the preparation to a medium containing a high divalent cation concentration, produced a temperature dependent failure of neuromuscular transmission. Failure of transmission developed in an "all-or-none" mode and was reversed by decreasing the bath temperature or divalent cation concentration. This blockage of the neurotransmission is attributed to a presynaptic block of impulse propagation.