Abstract 1. 1. The effect of dexamethasone and its interaction with morphine has been studied on transmurally-stimulated guinea-pig ileum preparation. 2. 2. Dexamethasone dose-dependently depressed the contractions of the ileum; this action has a rapid onset. 3. 3. Naloxone did not reverse the inhibitory effect of dexamethasone. 4. 4. When dexamethasone and morphine were given in combination the maximum reduction observed was equal to the sum of the single effect of two drugs given individually and naloxone antagonized only the inhibition induced by morphine. 5. 5. Proteic synthesis inhibitors did not modify the inhibition induced by dexamethasone. 6. 6. RU-38486, a glucocorticoid antagonist receptor, antagonized completely the inhibitory effect of DXM without affecting the inhibition induced by morphine showing that the effect of dexamethasone occurs by glucocorticoid receptors-mediated processes.