The concept that anesthesia and analgesia are distinct states and therefore are possibly mediated by different mechanisms is stressed. Analgesic nitrous oxide is shown to act at specific rather than non specific central nervous system sites, as well as having a large number of actions similar to morphine the classical opioid. This includes the fact that specific opioid antagonists attenuate the effects of both morphine and analgesic nitrous oxide. Evidence is also provided showing that nitrous oxide may be a partial agonist and that it may interact with the endogenous opioid system by the release of endogenous opioids, and/or by direct action at the mu, delta, sigma and kappa receptors.