Summary—1The action of ergocristine at α‐adrenoceptors was studiedin vivoin the pithed rat, andin vitroon the rat isolated vas deferens.2In the pithed rat, the pressor response to ergocristine was reduced competitively by yohimbine, but not by prazosin.3Ergocristine decreased the tachycardia elicited by electrical stimulation of the cardioaccelerator sympathetic nerves, this effect being antagonized by yohimbine.4At postsynaptic α1‐adrenoceptors, on the vas deferens of the rat, ergocristine antagonized the contraction induced by phenylephrine in a competitive manner (pA2= 7.85).5These results show that vasoconstriction due to ergocristine is mediated by α2‐adrenoceptors and that, in the rat, ergocristine acts as an α2‐adrenoceptors agonist, and an α1‐adrenoce