1. The effects of 5-hydroxytryptamine (5-HT) and morphine on the responses to acetylcholine and nicotine of isolated rabbit atria were studied. 2. 5-Hydroxytryptamine (10 ug/ml.) and morphine (20 jug/ml.) blocked the negative chronotropic and inotropic actions of acetylcholine.3. Nicotine (20 ,ug/ml.) produced stimulation of the atria, which was blocked by dichlorisoprenaline, morphine, 5-HT, bretylium and hemicholinium. Hemicholinium block was reversed by choline.4. In reserpinized preparations, nicotine produced inhibition of atria and this action was also blocked by atropine, 5-HT and morphine. Inhibition induced by nicotine was potentiated by physostigmine. 5. 5-Hydroxytryptamine (20 ug/ml.) produced stimulation of atria. This was blocked by bretylium and reduced by hemicholinium. Hemicholinium block was reversed by choline. 6. It is concluded that 5-HT in low concentrations acts as a weak agonist at the cholinoceptive receptors and therefore blocks the action of acetylcholine. Furthermore, nicotine and larger doses of 5-HT have actions on ganglionic structures and liberate acetylcholine, which in turn releases catecholamines.Although acetylcholine is known to have a negative inotropic action on isolated rabbit, atria, Hoffmann, Hoffmann, Middleton & Talesnik (1945) demonstrated that it produces stimulation in the presence of atropine and that this action is due to the release of catecholamines. Trendelenburg (1960) analysed the action of 5-hydroxytryptamine (5-HT) on isolated rabbit atria and postulated that 5-HT produced its effects through an action on neural elements in the atrial tissue. Jacob & Poite-Bevierre (1960) have shown that 5-HT has three successive phases of action on the isolated rabbit heart. The negative chronotropic and inotropic actions of 5-HT were shown to be blocked by atropine by these workers. In view of this, a study on the interactions of 5-HT and acetylcholine on isolated rabbit atria was undertaken. We have shown in the present investigation that 5-HT, in concentrations that fail to produce any discernible effects, influences the action of acetylcholine and nicotine on the atria.