Pretest administration of ACTH has been shown to produce recovery of an extinguished avoidance response. Presumably this effect is found because endogenous ACTH is a component of the original training memory. However, another possible explanation of this finding is that administration of the peptide acts as a novel stimulus that "disinhibits" the extinguished response. In order to test this "disinhibitory" hypothesis of ACTH-induced recovery of an extinguished avoidance response, some subjects were given dexamethasone 2 hr prior to training and extinction. This synthetic glucocorticoid is effective in blocking endogenous release of ACTH. Thus, ACTH should not be a component of the training memory in subjects given dexamethasone prior to training and extinction but would be a relatively novel stimulus condition at testing. Pretest administration of ACTH was found to be effective in alleviating performance deficits induced by extinction only for subjects given saline prior to training and extinction. Administration of ACTH had no effect on the avoidance responding of subjects given dexamethasone. These findings suggest that pretest administration of ACTH affects retrieval processes rather than acts as a disinhibitor.