In normal rat, anaesthetized with pentobarbital, 5-HT, injected or perfused by intravenous route, provokes different types of cardiovascular responses. These variations are due to the activation of three classes of receptors. The first ones explain the fall of the arterial blood pressure: they are located on arterioles which are dilatated. The second one are para- or perivascular nervous receptors: when stimulated, they induce vago-vagal reflexes (Bezold-Jarisch reflexes) suppressed by vagotomy. The third ones are vasoconstrictor and located on arterioles.