Regional myocardial dynamics were assessed during continuous, gradual coronary flow reductions caused by spontaneous thrombus formation in the stenosed left circumflex coronary artery of eight open chest dogs. Contractile changes in the subendocardial and subepicardial layers were measured by ultrasonic crystal techniques. Segment length shortening was continuously measured as coronary flow reductions occurred. Contractile dysfunction in the subendocardium preceded that in the subepicardium. At lowest flow levels the subendocardial function degenerated to severe holosystolic bulging whereas some systolic shortening was maintained in the subepicardium. Exponential equations derived to express changes in end diastolic segment length and end systolic length as a function of coronary blood flow for both subendocardium and subepicardium indicated that increases in end systolic length were a more sensitive index of ischaemia than increases in end diastolic length for both the subendocardium and subepicardium during coronary flow reduction. Comparison of exponential curves of end diastolic segment length vs coronary blood flow in subendocardium and subepicardium showed a small but significant difference (p less than 0.02). Comparison of exponential curves of end systolic length vs coronary blood flow in subendocardium and subepicardium showed a large difference in changes at end systole occurring between subendocardium and subepicardium (p less than 0.001). Active contraction of the subepicardium may serve to limit the extent of paradoxical systolic segment lengthening in the underlying ischaemic myocardium and thus help to preserve ventricular function during acute gradual coronary flow limitation.