Acyltryptophols reversibly inhibit muscle contractions caused by the actions of acetylcholine and raised potassium ion concentrations.

1. Acetylmethoxytryptophol, originally isolated from the pineal gland, inhibits both the nicotinic and muscarinic receptor stimulatory activities of acetylcholine on frog rectus muscle and guinea pig ileum. 2. Synthetic homologues and analogues, including the acetyl-, propionyl-, butyryl-, and valeryl-methoxytryptophols, tryptophols and alpha-methyl-tryptophols have been prepared and shown to possess similar activity on the frog muscle, with butyryl compounds being the most active. Methoxytryptophol, tryptophol, alpha-methyltryptophol and acetylhydroxytryptophol possess little or no activity. 3. All acyltryptophols tested inhibit the effect of increased potassium concentrations on frog muscle. 4. It is concluded that the acyltryptophols act not at the transmitter receptor level but either at the potassium ion channel or elsewhere in the cell membrane.