The effect of oxygen on local contraction and ST segments was evaluated in 15 dogs prior to and throughout two hours of coronary occlusion and two hours of reperfusion. After coronary occlusion, epicardial ST segments rose and ischemic and border zone total tension decreased similarly on either room air or on oxygen. After 15 minutes of reperfusion, five of eight dogs on room air and four of seven dogs on oxygen showed new Q waves. ST segments decreased consistently in all experiments. On room air, ischemic and border zone tension decreased further(p <0.01). In contrast, in the oxygen group neither ischemic nor border zone tension decreased further following reperfusion. Thus, 40% oxygen did not prevent contraction abnormalities after coronary occlusion. These data suggest that it may prevent a further decrease in zonal contraction following reperfusion.