Abstract
1. The effects of quazodine and theophylline have been studied on the rat hemidiaphragm and chick biventer cervicis preparations in vitro.2. Theophylline and quazodine enhanced maximal twitches and contractural responses to acetylcholine and carbachol.3. These actions on contractility were exerted directly upon the muscle fibres and were dependent upon the concentration of calcium ions in the bathing solution.4. In addition, quazodine enhanced the neuromuscular blocking activity of tubocurarine, probably by a prejunctional action.5. Theophylline potentiated acetylcholine to a greater extent than it potentiated carbachol thus suggesting that theophylline possesses anticholinesterase activity.