Stimulation of opioid systems with opiate agonists produce characteristic alterations in the investigatory behavior exhibited by rats in a novel environment. As numerous reports now indicate that opioid systems can be activated by exposure to stress, the following study examined whether exposure to stressors could produce opiate-like alterations of investigatory behavior. Naive rats were exposed to one of three stressors (restraint, tailpinch pressure, high intensity white noise) or to control procedures, and were observed in a novel environment. The frequency and duration of a wide range of behavioral activities were recorded. All three stressors were found to produce morphine-like alterations of investigatory behavior. The average time an animal spent per contact with stimuli in the environment was decreased significantly by stress, with greater reductions being associated with locomotor hypoactivity. The stress-induced reductions of investigatory behavior were blocked by very low doses of the opiate antagonist naloxone (0.1-0.25 mg/kg). These results are consistent with an activation of opioid systems underlying some of the changes in investigatory behavior produced by exposure to stress.