Two distinct binding sites for serotonin have been identified in brain tissue and labeled 5-HT1 and 5-HT2 binding sites. The receptor mediating contractions of several types of vascular smooth muscle (including the canine basilar artery), aggregation of blood platelets, and contractions of tracheal muscle can be defined as a 5-HT2 serotonergic receptor. The prejunctional inhibitory effect of serotonin on adrenergic neurotransmission is mediated by receptors which have the same affinity for agonists as the 5-HT1 binding sites. The other peripheral actions of serotonin, in particular its vasodilator effects and its excitatory effects on gastrointestinal smooth muscle, do not involve 5-HT2 serotonergic receptors.