1. A study is presented of the actions of certain inhibitors on the frog rectus abdominis muscle stimulated by acetylcholine.

2. A type of analysis has been developed which provides a reliable criterion for judging whether an inhibitor is competing with acetylcholine for receptors at the myoneural junction or whether acting by a different mechanism.

3. The "curares" are shown to act by competitive inhibition at the myoneural junction, confirming earlier work of others on the mode of action of curare.

4. Atropine acts as an inhibitor at the myoneural junction. The inhibition may be non-competitive or it may be complicated by an additional effect at some point other than the myoneural junction.

5. A possible mechanism for anomalous inhibitor effects is the action of a single compound at more than one locus in the Ach mechanism. Eserine exerts such a dual effect at the end-plate.

6. Some of the available electrical and chemical data have been correlated to make possible a partial explanation of the role of Ach in transmission at the myoneural junction.

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