Summary: Methysergide, dihydroergotamine, ergotamine and 1‐methylergotamine potentiated the response of both the human temporal and rabbit ear artery to noradrenaline. In higher doses, these drugs caused constriction of the rabbit arteries, but had little constrictor effect on the human vessels. Cyproheptadine and pizotifen did not cause either potentiation or direct vasoconstriction on either artery type. Pizotifen abolished the direct constrictor action of 5‐hydroxytryptamine (5HT) while leaving its potentiating properties unaffected; it also abolished the constrictor effects of methysergide but was without effect on the constrictor actions of the ergot derivatives. It is concluded that of the drugs tested only methysergide constricts the arteries by a direct action on 5HT receptors, and that 5HT, methysergide and the ergot derivatives have comparable activities at a separate receptor which is responsible for potentiation. It is suggested that, in the treatment of migraine, the therapeutic value of methysergide and the ergot derivatives, hut not cyproheptadine or pizotifen, may lie in their ability to replace 5HT as a sensitizing agent.