An understanding of receptors and mechanisms involved in pain and suffering is essential in selecting proper methods of treatment. Detection of pain, knowing types of pain, and understanding reasons why presurgical administration of analgesic drugs (pre-emptive analgesia) is essential to relief of pain and distress. Animals feel pain to the same extent as humans and they have a wide variation in tolerance to pain stimuli as well. Animals may express pain or discomfort in two ways. Excessive activity or relative lethargy are both overt reflections of a painful state. Expressions of pain may be exhibited by moaning, groaning, crying, whimpering, looking at the painful area, licking or biting or simply a decrease in activity. Pain may be categorized as minor, moderate, or severe. Pain may be superficial, somatic (muscle and skin), or visceral or combinations. Moderate and severe pain always require treatment and visceral pain is usually more intense than somatic pain. Opiate receptors are the normal sites of action of several endogenous substances. There are three major types of opioid receptors designated as mu, kappa and sigma. Opioids and opioid antagonists have different affinities for different receptors. Agonist analgesics such as morphine, meperidine, fentanyl, and oxymorphone exert their effects by attaching to the mu receptors. A relatively pure antagonist, naloxone, will displace another compound on these receptors but exert little or no effect. The mixed drugs, agonist–antagonists or partial agonists, may be used as analgesics if no opioid compound has been used, or as antagonists to reverse the effects of an opioid on the mu receptor while at the same time not competing with the effects on the k receptor that is, preserving the analgesia provided by that receptor. Some of the mixed drugs are better agonists wile others are better antagonists. Animals recovering from surgery are often surprised and confused because they hurt and therefore sedation along with analgesia preanesthesia is often desirable and necessary for proper treatment of postsurgical pain. Therefore, giving an analgesic before surgery will be advantageous. The second choice is to give analgesics after surgery, but before recovery from anesthesia.