A study was carried out to determine the effect of distal aortic constriction on the regional oxygen supply-consumption balance in ischaemic and non-ischaemic myocardium. Left anterior descending coronary artery flow was reduced by 50% in anaesthetised mongrel dogs, and half these animals were subjected to distal ascending aortic constriction (ventricular systolic-aortic systolic pressure gradient 50 mm Hg). Blood flows were determined using radioactive microspheres, and arterial and venous oxygen saturations were determined using microspectrophotometry. When left anterior descending artery flow was decreased by 50% tissue flow, as measured by microspheres in the ischaemic region in both control and aortic constricted animals, decreased. Aortic constriction increased flow in the non-ischaemic zone compared with the same region in control animals. The arteriovenous oxygen content difference increased similarly in the ischaemic region in control and aortic constricted animals compared with their respective non-ischaemic zones. Aortic constriction decreased this difference in the non-ischaemic region compared with these same regions in control animals. Oxygen consumption in the non-ischaemic and ischaemic regions of aortic constricted animals was higher than in control animals. The oxygen supply to consumption ratio was lower in the ischaemic zone in both control and aortic constricted animals than in their respective non-ischaemic regions. Aortic constriction does not appear to alter the oxygen supply-consumption balance in the ischaemic myocardium compared with control animals. Thus the increased work performed by the heart in response to the aortic constriction is adequately matched by an increased flow even in the flow restricted region.