RECENT experiments1 on the synthesis of acetylcholine by tissue extracts, especially from acetone dried brain, human placenta, and horse spleen, have shown that flavine adenine dinucleotide probably plays an essential part. It was found that spleen extracts only formed small amounts of acetylcholine under conditions giving a large synthesis with brain and placental extracts. Saline extracts of acetone dried spleen, however, were found to contain a powerful inhibitor of acetylcholine synthesis. The effect of the inhibitor, which has the properties of an enzyme, was counteracted by adding large amounts of flavine adenine dinucleotide. This suggests that the inhibitor is an enzyme which breaks down flavine adenine dinucleotide ; the action of tissue extracts on the dinucleotide has therefore been investigated in more detail.