Excitatory Pavlovian conditioning of a discrete CS is attenuated by prior exposure to the UCS. The UCS preexposure phenomenon is observed in a variety of Pavlovian conditioning procedures as diverse as eyelid conditioning, the conditioned emotional response, and conditioned taste aversion learning. This article discusses the variables that affect the UCS preexposure phenomenon and uses this information in evaluating both associative and nonassociative accounts of the phenomenon. At least one associative account, based on context blocking, and at least one nonassociative account, based on central habituation of the emotional response to the UCS, remain viable. (71 ref) (PsycINFO Database Record (c) 2006 APA, all rights reserved). © 1979 American Psychological Association.