Five dogs had aorto-coronary bypass with autogenous jugular vein and six had coronary bypass with internal mammary artery (IMA). Cineangiography 8 to 10 months later in dogs with vein grafts demonstrated irregularity and narrowing of the grafts while 12 to 22 months later the IMAs were smooth and patent. All dogs were studied on right heart bypass and one minute of graft occlusion produced similar changes in reactive hyperemia, cardiac hemodynamics, and myocardial metabolism in both groups. Gross and microscopic study of the grafts revealed generalized intimal fibrous proliferation in all vein grafts which was focally severe and produced anastomotic stenosis in two. Intimal proliferation did not occur in IMA grafts. These observations support the use of the IMA for coronary reconstruction in man.