exploratory locomotion occurred more gradually when the social defeat sessions were 72 hours apart than when the social defeat sessions occurred daily. This suggests that the intermittent stress regimen was not as efficient at inducing these behavioral changes, although the overall magnitudes of the behavioral effects were eventually roughly equivalent. Once again, the intruders' behavioral adaptations did not decrease the displays of agonistic behavior from the residents (consistent number of defeats and equivalent latencies to first defeat across groups and days). The equivalence in basal CORT concentrations as well as the equivalence in glandular masses between the chronically stressed animals and the control animals indicates that the temporally spaced regimen was not potent enough to sufficiently alter hormonal or glandular basal states. Even though the more temporally spaced chronic stress regimen was able to produce equivalent effects with regards to behavioral changes (albeit, more slowly), it was less effective than the daily regimen in producing the hormonal and glandular changes that correlate with some of the symptoms of maj or depression. The results indicate, therefore, that there may be a critical window for vulnerability to an additional stressor--it is possible that the rats were able to partially recover from the initial stressor by the next exposure in the more temporally spaced chronic regimen. Relevance to Previous Work Overall, these results confirm and extend a previous report that daily social defeat is an effective emotional stressor for male rats. In a study by Haller and colleagues (1999), intruder rats were exposed to resident rats for 4 hours on four consecutive days, producing increased basal CORT concentrations in the intruders. The daily social defeat stress regimen used in our study effectively increased basal CORT concentrations as