The specific aim of this study and the hypothesis it tests are as follows: To determine the extent to which subjective analgesic effects of VR analgesia are inhibited by opioid receptor antagonism with naloxone. Hypothesis - VR analgesia will not be inhibited by systemic opioid receptor antagonism, suggesting that VR analgesia is not mediated by release of endogenous opiates and/or by activation of opioid-dependent descending central nervous system pathways.