Ergot alkaloids increase the tone of isolated canine vein strips dose-dependently in concentrations considerably lower than noradrenaline, but the maximal responses are only about one third of those to noradrenaline. This action can be blocked by prior administration of phentolamine. It is therefore concluded that the long-lasting stimulant action of ergot alkaloids on vascular smooth muscle is mediated mainly by alpha-adrenoceptors. Using the autoperfused hind limb of the cat, it has been shown that dihydroergotamine increases dose-dependently the tone of the capacitance vessels in a manner very similar to electrical stimulation of the sympathetic nerve with increasing frequencies. However, in contrast to sympathetic nerve stimulation, dihydroergotamine elicits only a very weak increase in arteriolar resistance.