1 In healthy male volunteers dazoxiben (UK 37248), an inhibitor of thromboxane synthetase, abolished the arterial and venous vasoconstriction produced in the forearm by cold stimulation. 2 Aspirin alone had no effect on this vasoconstriction but negated the effect of dazoxiben. 3 In vitro dazoxiben increased the production of prostacyclin by human arteries and veins. 4 By reducing the synthesis of thromboxane A2, a potent vasoconstrictor, and by increasing the synthesis of prostacyclin, a potent vasodilator, dazoxiben may have a therapeutic role in conditions associated with abnormal vasoconstriction.