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These experiments explored the effects of non-contact exposure to two rodent species (rats and gerbils) on the defensive behaviors of male CD-1 mice. In Experiment 1 it was found that rats evoked substantial levels of defensiveness which were positively related to increases in rat activity. This effect was considerably attenuated, but still detectable, 30 min after rat exposure (Experiment 2). In Experiment 3 the effectiveness of rats and gerbils were compared. Gerbils were found to evoke a more modest increase in defensiveness than rats. These experiments indicate that non-contact exposure to rodents displaying varying levels of activity and, perhaps, propensity for predatory behavior provide a useful level of experimental control over the intensity of fear-like responses.