The present study investigated whether the anticipatory heart rate (HR) deceleration response may reflect a pre-attentive process of stimulus registration and how reaction time (RT) and HR responses are influenced by the introduction of a psychological stressor. 60 subjects participated in a signalled RT task with a feedback stimulus containing information on their reaction time and accuracy. Changes in HR, skin conductance (SC) and respiration activity were monitored during performance in two conditions of a visual stimulus recognition task with a fixed foreperiod. In one condition subjects were informed that some electric shocks would be delivered to their right wrist (stress condition); in the other, subjects were simply engaged in the stimulus recognition without the stressor (no-stress condition). Stimuli consisted of geometrical figures and for each trial subjects were required to determine whether a probe stimulus was the same as or different from one of two memory items. Two reliable anticipatory HR decelerations, one preceding the imperative stimulus and the other preceding the feedback signal, were observed. Because the HR deceleration preceding the feedback signal (that did not require the inhibition of any specific motor response) was more pronounced than that obtained for the probe stimulus, it was concluded that HR deceleration response is an expression of stimulus processing rather than response preparation. Reaction times for 'same' stimuli were shorter than for 'different' stimuli. Averaged respiratory activity showed that with the onset of a warning signal subjects inspired and held their breath until they received the feedback signal. The averaged skin conductance data showed two main phasic increases, one after the probe stimulus onset and the other after the delivery of the feedback signal. This was taken to reflect the orienting response to the most significant stimuli. © 1995.