Post-traumatic stress disorder (PTSD) is an anxiety disorder that develops after experiencing a significant physical and/or psychological trauma. Frequently, people experiencing such traumas can associate certain stimuli with the traumatic event such that future exposure to the stimuli triggers intense fear and helplessness. It has been shown that acute exposure to morphine, which stimulates the endogenous opioid system, immediately following a traumatic event can prevent PTSD. In line with this, post-training acute injections of morphine also blocks Pavlovian fear conditioning, an animal model of PTSD. However, no previous study has examined the effects of acute morphine in fear conditioning in female rats. Since women tend to be more susceptible to PTSD than men, examining the mechanisms underlying fear conditioning in females might lead to better understanding of the development of PTSD, and to better characterize sex-related differences in the involvement of the opioid system in fear learning. In the present study, we will use auditory and contextual fear conditioning to determine whether acute morphine exposure can differentially decrease fear conditioning between male and female rats. We hypothesize that acute morphine will have a stronger effect in reducing fear conditioning in female rats as compared to male rats. Results from this study could provide useful information to prevent the development of PTSD in a sex-specific manner.