The aim of this study was to investigate in normal subjects the effect of a cold pressor test on the caliber of the radial artery, a muscular artery of medium size. The internal diameter of this artery was measured continuously using a recently developed ultrasonic device. Immersion of one hand in ice water for two minutes increased blood pressure from 115/75 +/- 3/2 (Mean +/- SEM) to 136/90 +/- 6/2 mm Hg (P less than .001) and decreased the internal diameter of the radial artery from 2.82 +/- 0.12 to 2.60 +/- 0.09 mm (P less than .01). These data therefore indicate that the vasoconstriction induced by the cold pressor test involves not only arterioles, but also medium-size arteries.