We hypothesized that autonomic responses conditioned to fear-relevant stimuli, in contrast to responses conditioned to neutral stimuli, can be elicited after only an automatic, non-conscious analysis of the stimulus. Consequently, they may be expected to be insensitive to verbal instructions. Normal subjects were conditioned to either fear-relevant stimuli (snakes or spiders) or neutral stimuli (flowers or mushrooms) in a differential conditioning paradigm with shock as the unconditioned stimulus. In a subsequent extinction series, half of the subjects were shown the conditioned stimuli under masking conditions preventing their conscious recognition, whereas the other half were exposed to non-masked stimuli. Then half of the subjects in each of the masking conditions were verbally instructed that no more shocks would be delivered and then the extinction trials followed. Consistent with our hypothesis, differential responses to the fear-relevant CSs+ and CSs- remained unaffected by both masking and instruction, whereas differential responding to neutral stimuli was wiped out by the masking procedure and the verbal instruction.