The purpose of this experiment was to study the possible modulatory role of the bed nucleus of stria terminalis (BNST) in the regulation of affective defense and quiet biting attack reactions in the cat. The experimental paradigm employed concurrent electrical stimulation of the hypothalamic attack sites and of the BNST. The results of the present study demonstrate that concurrent electrical stimulation of the BNST can differentially modulate the two different forms of aggressive behavior by facilitating affective defense and by suppressing quiet biting attack.