In 4 experiments with 204 male CD-1 mice, exposure to inescapable shock disrupted performance in both shock- (SE) and water-escape (WE) tasks. These deficits were prevented in Ss that were previously trained in the same task. However, an asymmetrical immunization effect was seen in a cross-stressor paradigm. Whereas deficits of WE performance engendered by inescapable shock were prevented by prior SE training, the deficits of SE were not eliminated by prior WE training. Evidently, the immunization effect occurs when initial training and subsequent testing are conducted in the same task or when the initial training and uncontrollable stress session involve the same aversive stimulus. Norepinephrine (NE) determinations revealed that reductions of NE introduced by inescapable shock were unaffected by prior SE training and were enhanced by prior exposure to the stress of water immersion. Thus, although the performance deficit introduced by inescapable shock may be related to variations of NE, the immunization effect probably was unrelated to alterations of NE. Data provisionally suggest that the immunization stems from 2 independent factors: Initially training Ss in an active escape task may (a) disrupt subsequent learning that the inescapable stress actually is uncontrollable and (b) limit the influence of the motor deficits introduced by uncontrollable shock on subsequent escape performance. (28 ref) (PsycINFO Database Record (c) 2006 APA, all rights reserved)