The response of limb blood flow to cold stimulus was determined by venous occlusion technique in 30 healthy subjects. The stress was applied by immersing one hand into ice-floating water for 35 sec, and the blood flow was measured serially in the contralateral upper limb. The change of blood flow at the 15th sec of stress was the largest among the intermittent measurements, and decreased by 48+/-8% of the control value. A significant rise in plasma dopamine-beta-hydroxylase was found in response to the same stress. This simple cold stress test may be used to evaluate the function of the reflex arc involved in reflex vasoconstriction. A significantly diminished vasoconstriction was observed in 12 uremic patients with a concomitantly smaller rise of plasma dopamine-beta-hydroxylase activity in the test.