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The action of the anticholinesterase Paraoxon on neuromuscular transmission in rat diaphragm and mouse omohyoideus preparations was investigated. In both preparations, when Paraoxon potentiated the twitch in response to a single nerve stimulus, repetitive muscle action potentials were recorded with an intracellular electrode placed at the motor end-plate region. At end-plates of Paraoxon-treated rat diaphragm preparations where the membrane potential was not sufficiently negative to support muscle action potentials, repetitive end-plate potentials were recorded in response to a single nerve stimulus. No repetitive end-plate potentials could be recorded under such conditions in preparations which had been exposed to dithiothreitol before being treated with Paraoxon, although twitch potentiation and repetitive muscle action potentials were still observed in these preparations. In Paraoxon-treated mouse omohyoideus preparations only single end-plate potentials were recorded from end-plates where the membrane potential was not sufficiently negative to support muscle action potentials. This applied whether or not the preparation had been treated with dithiothreitol before being exposed to Paraoxon. In voltage-clamped rat diaphragm preparations which had been treated with Paraoxon, repetitive end-plate currents were frequently recorded in response to a single nerve stimulus. Under the same conditions mouse omohyoideus preparations responded with a single end-plate current. It is concluded that Paraoxon-induced twitch potentiation in rat diaphragm and mouse omohyoideus preparations is caused by repetitive muscle action potentials being triggered by a single nerve stimulus. Under the conditions stated, the repetitive muscle action potentials in rat diaphragm preparations arose from a prolonged end-plate potential or repetitive end-plate potentials or a combination of both. In mouse omohyoideus preparations the repetitive muscle potentials were the consequence of a single prolonged end-plate potential.