This experiment is concerned with evaluating the cardiac responses of human Ss in an aversive situation. Two main conditions were investigated: Control of shock occurrence (Avoidance) and Passivity, that is, Ss were instructed to passively receive the shocks. Sixty Ss were used. The Avoidance condition was further divided into 3 specific groups. One group of Ss had to avoid the aversive stimulus by a proper task (depressing keys) without any certainty about the result of their action until the habitual moment of shock occurrence. Ss in a second group were provided immediate feedback from their action via a light bulb. A third group of Ss kept receiving shocks whatever they tried. After completion of 30 trials the conditions for each Ss were reversed by appropriate instructions and run for another 20 trials. A decelerative cardiac response was found in anticipation of shock for Ss in the Passivity condition. In the Avoidance condition, such a deceleration was found for trials during which Ss had had a feedback of their failure to avoid (second group) and were expecting an oncoming shock. Furthermore, only the Ss in Avoidance condition displayed HR acceleration after the signal onset of the ISI. Specific variations were found in the second phase of the experiment for the third group of Avoidance condition.