We have previously reported (EPW, SFN Abstract, 1996, 97, 99) evidence for a spinal opiate mechanism in the hyperalgesic state (tailflick test) that results from the intraperitoneal administration of lipopolysaccharides (LPS; pyretic) or lithium chloride (LiCl; emetic), through the pairing of a unique flavor with illness, i.e., conditioned hyperalgesia, and the subcutaneous injection of formalin (dorsum of hindpaw). The goal of the present experiments was to examine supraspinal opiate involvement in LiCl- and formalin -induced hyperalgesias in rats previously implanted (one to two weeks earlier, to allow for recovery) with intracerebroventricular (ICV) cannulae (3rd ventricle). Here, the ability of ICV injection of specific opiate antagonists (CTOP, Naltrindole, Nor-Binaltorphimine) and naltrexone to dose-dependently block LiCl- and formalin-induced hyperalgesias in the tailflick test was compared to ICV vehicle controls. In contrast to vehicle controls, ICV injections of naltrexone (10, 1.0, and 0.1 ug) b...