In rats, an animal model of individual differences in emotional reactivity has been developed. When outbred rats are exposed to a novel environment, some rats exhibit high locomotor response (HR), while others exhibit low rates of locomotor activity (LR).2 Stress-induced locomotion in a novel environment predicts subsequent behavioral responses of these animals to drugs of abuse.2 Therefore, HR rats exhibit higher rates of amphetamine- and cocaine-induced locomotor activity and will self-administer these drugs more than LR rats.2,3 In addition, HR rats seek novel and varied environments when given a choice between these environments and environments to which the rats have become habituated.4 Furthermore, compared with LR rats, HR rats show less anxiety-like behavior in tests such as the light-dark box and the elevated plus maze.5 The light-dark test of anxiety is based on the rats' fear of a bright environment. The elevated plus maze test of anxiety is based on the rats' fear of open and elevated surfaces. In both conditions, HR rats are more apt to explore the bright environment and visit the open elevated arms of the maze compared with LR rats.5 Interestingly, the HR rats, while appearing less anxious in novel situations, release high levels of the stress hormone, corticosterone, into their blood streams.4,5 It is conceivable that the increased magnitude of stress-induced plasma glucocorticoid secretion in the HR animals plays a role in determining individual differences in expression of novelty-seeking behavior, with HR animals finding the increased glucocorticoids rewarding. Indeed, Piazza et al6 have shown that HR rats self-administer corticosterone more readily than LR rats. However, while HR rats may appear less anxious, they are sensitive to changes in their environmental conditions. For example, it has been shown that a week of social isolation makes HR rats behave like LR rats in an anxiety test.5 In another example, it was possible to inhibit HR rats' cocaine self-administration following chronic social defeat.3 It is therefore simplistic to categorize HR rats as less anxious and LR rats as more anxious, or HR rats as drug seekers and LR rats as non–drug seekers. Behavior depends on the environmental conditions, the stressor severity, and the animal's coping mechanisms.