The main goal of this study was to assess cortical functioning as indexed by the postimperative negative variation (PINV) induced by uncontrollable stress. Sixty-six persons were randomly assigned to three groups who underwent different sequences of stressor controllability. Within an S1-S2 paradigm, one group had initial control over aversive stimulation followed by loss of control and restitution of control. The other groups initially experienced either uncontrollability or controllability followed by a condition of control. Uncontrollable stress significantly enhanced PINV magnitudes independent of preceding control. However, control over aversive stimulation prior to loss of control normalized PINVs during restitution of control. Persons not experiencing prior control showed enhanced PINVs, longer reaction times, and more errors during restitution of control. We conclude that cortical activation changes are linked to the evaluation of instrumental contingencies. However, the exact determination of brain regions involved in the processing of uncontrollable stress needs further investigation.