To explore possible mechanisms of left ventricular early segmental relaxation, complete occlusion of the left anterior descending coronary artery (LAD) was produced in seven open-chest dogs and partial occlusion of the LAD was produced in six open-chest anesthetized dogs. Regional wall thickness was measured both in an ischemic and a normally perfused zone using implanted ultrasonic crystals. Two to three seconds following complete LAD occlusion, thinning of the ischemic wall occurred prematurely during isovolumic relaxation. The extent of premature thinning became more prominent 5 to 10 sec following LAD occlusion. Early thinning of the ischemic wall preceded thinning of the normally perfused wall by 110 +/- 10 msec. Partial occlusion of the LAD produced a 33 +/- 6% reduction of coronary flow and a 23 +/- 4% reduction of systolic wall thickening in the ischemic region. Systolic thickening of the nonischemic wall was unchanged relative to the preocclusion period. Premature early thinning of the mildly ischemic wall preceded thinning of the normally perfused segment by 90 +/- 8 msec. The observation that ischemia can produce segmental early thinning of the ventricular wall may have implications in understanding the mechanism of the angiographic observation of the segmental early relaxation phenomena in patients with coronary artery disease.