The effects of a DAi dopamine receptor selective agonist, fenoldopam, were examined on human blood vessels. Fenoldopam relaxed precontracted human arteries in vitro from brachial, cerebral, cervical, colic, coronary, lumbar, pulmonary, renal and splenic sites. Fenoldopam failed to relax uterine or external iliac arteries or saphenous vein segments. These effects of fenoldopam were not endothelium dependent, but were antagonised by the selective DAi receptor antagonists, SCH 23390, (+)-butaclamol, and (R)- more than (S)-sulpiride. The effect of fenoldopam may involve cyclic adenosine monophosphate. These studies indicate the presence of DAi receptors on a variety of large isolated human arteries.