The effect of serotonin on the blood pressure and the heart rate of the pigeonColumba livia was investigated in anaesthetised condition. The effect of histamine was also investigated to understand the mechanism of action of serotonin. Serotonin and histamine were both depressors. Serotonin produced bradycardia while histamine produced tachycardia. Atropine treatment reversed the serotonin-induced depressor response but only partially blocked the histamine-induced fall in blood pressure. Atropine treatment completely abolished the serotonin-induced bradycardia but partially inhibited the histamine-induced tachycardia. A reflex vagal activity and a direct vasoconstrictor activity were proposed on the action of serotonin.