Dihydroergocristine (DHEC) and dihydroergotamine (DHE) were investigated on canine saphenous veins in vivo and on canine saphenous veins and basilar arteries in vitro. Following local i.v. infusion in vivo, the venoconstrictor response to DHEC was about 30% weaker than that produced by DHE. When administered orally, however, both ergot alkaloids elicited similar venoconstrictor effects. In vitro maximal contractile responses to DHEC and DHE of basilar arteries were only 20-30% of those produced by 5-HT, whereas in saphenous veins both DHEC and DHE elicited similar maximal effects as those observed with 5-HT. In saphenous veins, methiothepin antagonized venoconstrictor responses to 5-HT, DHEC, and DHE within the same concentration range, being significantly less potent when tested against noradrenaline. The reverse was true for yohimbine, which was significantly more potent against noradrenaline than against 5-HT, DHEC, and DHE. It is suggested that the venoconstrictor responses to both DHEC and DHE are mediated through 5-HT1-like receptors.