Morphine (0.05 to 0.1 mug./ml.) markedly inhibited the contractions of the isolated guinea-pig ileum caused by nicotine, barium, and 5-hydroxytryptamine while the actions of acetylcholine, carbachol, and histamine were affected only a little. Atropine (0.025 to 0.05 mug./ml.) had a similar effect, in addition to its known effects on acetylcholine and carbachol contractions. Morphine had no additional effect on the inhibitory action of atropine, while in the presence of morphine, atropine had a significant additional inhibitory action only on the contraction caused by nicotine.The action of barium was complex. It caused contractions of a rhythmic type alternating with relaxations, a pattern which is similar to that produced by the emptying phase of the peristaltic reflex; these oscillations were inhibited by hexamethonium. On the other hand, that part of the sustained barium contraction which was inhibited by morphine may be explained by an action on the nerve cells innervating the muscle fibres (motor neurones). Similarly, the morphinesensitive component of the 5-hydroxytryptamine (5-HT) contraction was probably also due to an action on the motor neurones. The morphine-insensitive contractions of barium, 5-HT, and nicotine were believed to be caused by a direct action on the muscle fibres.The morphine inhibitors, nalorphine and levallorphan, had different effects with the different agonists. Their morphine-like action was particularly pronounced on the effect of 5-HT, which they antagonized, while their morphine-protecting action was most strongly present on the effect of nicotine.