Used 3 types of UCS: shock paired with light offset, light offset alone, and shock alone. Sensitization controls were run for the first and third types. The groups that had shock as part of the UCS showed conditioning and extinction, the other groups did not. Concluded that reinforcement effects conditioning of an autonomic nervous system response. The equivocal results of previous work in this area may be due to the absence of reinforcement. Suggest that in the present study a generalized fear reaction was modified rather than a specific light reflex. 19 references. (PsycINFO Database Record (c) 2006 APA, all rights reserved)