This project combines two areas of interest in the study of classical conditioning. The first of these is blocking, or the finding that when a compound of two conditioned stimuli, CS1+CS2, is presented paired with an unconditioned stimulus (UCS), subjects will show a lesser response to CS2 when presented alone if CS1 was presented paired with the UCS prior to the presentation of the compound, presumably because CS2 carries no additional information about UCS occurrence than CS1 already does. The second area of interest is that of biological preparedness, or the idea that there are certain stimuli of a fear relevant nature that seem to follow different patterns of conditioning than other stimuli, in that they develop conditioned responses that take longer to extinguish and are unaffected by cognitive expectancy or awareness of the CS-UCS relationship. This study seeks to examine the outcome when these stimuli are used in a blocking procedure. 37 undergraduate students participated in a classical discrimination conditioning blocking paradigm. Differences in skin conductance responses (SCRs) to visual stimuli and an aversive shock UCS were measured in three groups, categorized as having neutral, fear relevant, or exciting CS2s. We found that a significant blocking effect did occur for the neutral group while blocking did not occur for the group presented with the fear relevant stimuli, although these groups had equal awareness that CS2 predicted the UCS. These data suggest further evidence that these biologically prepared stimuli may follow different learning patterns than other stimuli.