We examined the expression, distribution, and contribution to vasodilatation of the calcium-activated potassium (KCa) channel family in the commonly used coronary artery bypass graft internal thoracic artery (ITA) and saphenous vein (SV) to understand the role of large conductance KCa (BKCa), intermediate-conductance KCa (IKCa), and small-conductance KCa (SKCa) channel subtypes in graft dilating properties determined by endothelium-smooth muscle interaction that is essential to the postoperative performance of the graft. Real-time polymerase chain reaction and western blot were employed to detect the messenger RNA and protein level of KCa channel subtypes. Distribution of KCa channel subtypes was examined by immunohistochemistry. KCa subtype-mediated vasorelaxation was studied using wire myography. Both ITA and SV express all KCa channel subtypes with each subtype distributed in both endothelium and smooth muscle. ITA and SV do not differ in the overall expression level of each KCa channel subtype, corresponding to comparable relaxant responses to respective subtype activators. In ITA, BKCa is more abundantly expressed in smooth muscle than in endothelium, whereas SKCa exhibits more abundance in the endothelium. In comparison, SV shows even distribution of KCa channel subtypes in the 2 layers. The BKCa subtype in the KCa family plays a significant role in vasodilatation of ITA, whereas its contribution in SV is quite limited. KCa family is abundantly expressed in ITA and SV. There are differences between these 2 grafts in the abundance of KCa channel subtypes in the endothelium and the smooth muscle. The significance of the BKCa subtype in vasodilatation of ITA may suggest the potential of development of BKCa modulators for the prevention and treatment of ITA spasm during/after coronary artery bypass graft surgery. Copyright © 2019 The American Association for Thoracic Surgery. Published by Elsevier Inc. All rights reserved.