Intrathecal microinjections of high doses of morphine in the rat produce algesia, hyperalgesia and hypersthesia as opposed to the analgesia found at lower doses. This hyperalgesia is not naloxone reversible and is only partially stereospecific. Morphine-3-glucuronide produces the hyperalgesia at low doses. This morphine-hyperalgesia may be mediated by a class of receptors similar to that which produces opiate-induced central excitation.