Opioids are considered the best type of pain medication for moderate to severe pain in cats after undergoing surgery or trauma. Common opioids include morphine, buprenorphine, hydromorphone and fentanyl. When an opioid binds to an opioid receptor, it causes an analgesic (pain alleviating) response. Buprenorphine is deemed preferable over other opioids because its effects last longer and it has limited side effects. Although buprenorphine binds very strongly to opioid receptors in the central nervous system, it does not achieve a maximal analgesic response. Therefore, it may be necessary to supplement use of buprenorphine with a more powerful opioid. However, the unique binding behavior of buprenorphine might leave more powerful opioids, such as fentanyl, relatively ineffective if the two drugs are used together. The purpose of this study is to investigate the analgesic interaction between buprenorphine and fentanyl in cats. The hypothesis is that pre-treatment with buprenorphine will decrease the pain relieving action of fentanyl. Researchers will also investigate the interaction between hydromorphone and fentanyl. Hydromorphone and fentanyl have a similar binding affinity to the opioid receptor and expect an increased pain relieving effect when these drugs are combined. Pain relief will be tested using thermal and mechanical pain thresholds as a direct indicator of pain. This method is a humane method of assessing pain and is in accordance with Winn’s humane policies. An increase in pain threshold to a thermal or pressure stimulus will be used to give an adequate representation of clinical analgesia. These findings will be used to guide veterinarians in drug selection for pre-surgical pain management or treatment for trauma in cats.