To assess the role of segmental dyssynchrony as a determinant of ischemic diastolic dysfunction, systolic and diastolic mechanics of ischemic and nonischemic myocardium were compared in the open chest pig heart (n = 7). Pacing tachycardia (1.8 x heart rate at rest) was imposed for 3 to 5 min in the presence of a single critical stenosis of the left anterior descending artery (demand ischemia, n = 7). After 30 min of recovery, the left anterior descending artery was totally occluded for 1.5 min in the same pigs (primary ischemia, n = 6). Both demand and primary ischemia increased left ventricular end-diastolic pressure and prolonged the time constant of left ventricular pressure decline. Percent systolic shortening of ischemic segments (perfused by the left anterior descending artery) decreased by 32% during demand ischemia and by 120% during primary ischemia, but that of nonischemic segments (perfused by the left circumflex artery) did not change significantly during either type of ischemia. During demand ischemia (but not during primary ischemia), left ventricular diastolic pressure increased relative to segment length so that a higher diastolic pressure was needed to stretch the ischemic segment to the same length (decreased distensibility). In nonischemic areas, diastolic pressure and segment length increased commensurately during both types of ischemia, indicating no change in diastolic distensibility. During demand ischemia, peak early diastolic lengthening rates increased in nonischemic segments but remained unchanged in ischemic segments. Diastolic segmental dyssynchrony developed during both types of ischemia, but was more pronounced during primary ischemia. Therefore, segmental dyssynchrony is unlikely to account for the rise in diastolic pressure relative to segment length seen during demand ischemia.