Placing animals in an unfamiliar environment triggers at least two major reactions: (1) a heightened state of arousal, fear, or distress and (2) a sharp increase in information processing as the animal attempts to learn about its new environment. These changes could have a profound effect on the way in which the animal reacts to the types of extraneous innocuous stimuli typically used to study learning and memory. For example, an increase in arousal or fear could either (1) make the animal more "attentive" to stimulus change resulting in a larger orienting response, or (2) produce a shift from orienting to defensive responding. Conversely, processing of the new stimuli present in the unfamiliar environment may make the animal less responsive to additional extrinsic stimulation. These possibilities were examined experimentally using both autonomic and behavioral measures of orienting and defensive responses. The results demonstrated that animals fail to exhibit either an orienting response or a defensive response to a novel auditory stimulus when they are first placed in an unfamiliar environment. With continued exposure to the test environment the orienting response appears and then shows a time-dependent increase in magnitude. This pattern of results was obtained in both preweaning and young adult rats. On the basis of additional research and analysis, it was concluded that a limitation in information processing capacity was the primary reason for the failure of the orienting response to occur when an animal is first placed in an unfamiliar test chamber.