Summary:The direct vasomotor effects of a number of agents used in the therapy of cerebral circulatory and metabolic disease were studied in isolated segments of cat pial vessels in vitro, Studies were repeated eight times gener ally, thus permitting the calculation of the mean maximum response-termed ifenprodil > vincamine > nicergoline > dihydroergotoxine > raubasine, In addition, it was considered worthwhile to determine whether the ifenprodil induced vasoconstriction occurred when human, rather than cat, pial vessels were studied, Ifenprodil and vincamine contracted the human vessels in a biphasic manner; the EC,;o was calculated to be 2,0 and 30 JLM, Based on the above observations, the following comments would appear justified, Firstly, an increase in cerebral blood flow does not necessarily mean a compound is a direct vasodilator. Secondly, the vasoconstrictory actions of some agents cor relate well with their anti-ischaemic properties (e,g" ifenprodil, vincamine, and nicergoline), Lastly, one action of effective anti-ischaemic agents might be to reduce flow, by vasoconstriction, in hyperaemic tissue: the inverse "steal" effect. Key Words: In vitro -a-adrenolytic agents-Middle cerebral artery Tension-Cat-Human.