Both the S-T segment shift and the injury current were measured using the direct-current magnetocardiogram (d-c MCG) in seven dogs undergoing coronary artery occlusion. The purpose of the measurements was to clarify the origin of the S-T shift in acute ischemia and infarction. Previous measurements, consisting of d-c electrograms recorded from the exposed epicardial surface in situ, are partially inconsistent; also, they are not necessarily representative of the surface electrocardiogram (ECG), which sums broadly over the myocardium. The d-c MCG allows steady myocardial currents in the intact torso to the measured externally; because the d-c MCG sums broadly over these currents, conclusions drawn from it are applicable to the ECG. Coronary artery occlusion was produced by inflating a tube which, about 1 week earlier, had been surgically installed around the artery and exteriorized. During occlusions carried out in the MIT magnetically shielded room, a sensitive magnetometer recorded the d-c MCG at various locations around the torso. Within 20 seconds after occlusion, equal and opposite S-T segment and base-line (d-c) shifts appeared on the d-c MCG; these shifts were maintained for at least 15 minutes, after which they slowly decreased. Therefore, during the acute ischemia produced by these occlusions, the S-T shift is a secondary result of a primary injury current that is interrupted during the S-T interval.