Satisfactory results were obtained with the radial artery used as a conduit for coronary artery bypass. However, spasm of this conduit was observed. Human radial and internal mammary artery ring segments were studied in organ chambers. Potassium chloride, norepinephrine, serotonin and thromboxane A2 mimetic were used to obtain dose-response curves, permitting assessment of force of contraction and sensitivity. The radial artery presents stronger contractions than the internal mammary artery. The two vessels have equal sensitivity to the vasoconstrictors used. These data emphasize the hyperreactivity of the radial artery and the need for prevention of vasospasm when this vessel is used as a conduit for coronary artery bypass.