1. The relationship between tremor and change in brain acetylcholine concentration after the injection of tremorine or oxotremorine has been investigated in rats.2. Tremorine produced a significant increase in whole brain acetylcholine and in tremor 5 min after injection. After this time tremor subsided but brain acetylcholine continued to increase.3. Oxotremorine produced tremor within 30 sec. This became maximal within 5 min of injection and then declined rapidly. The brain acetylcholine concentration showed a significant increase 5 min after injection and continued to increase until 30 min afterwards.