The aim of this study was to investigate the neuroanatomical regions implicated in the expression of the physical signs of morphine withdrawal by using local intracerebral injections of methylnaloxonium in dependent rats. Guide cannulas were implanted and aimed at the lateral ventricle, nucleus accumbens, central amygdala, anterior hypothalamus, medial thalamus, periaqueductal gray matter, locus coeruleus and nucleus raphe magnus. After surgery, rats were made physically dependent by s.c. implantation of two 75-mg morphine pellets. Methylnaloxonium, a quaternary derivative of naloxone (31-1000 ng), was administered 72 hr after pellet implantation. Methylnaloxonium administered i.c.v. induced a withdrawal syndrome similar to systemic naloxone, although several signs such as diarrhea, salivation, lacrimation and rhinorrhea did not appear, suggesting possible peripheral mediation. The most sensitive site for methylnaloxonium-precipitated withdrawal was the locus coeruleus. Signs such as jumping, rearing and locomotor activity were particularly frequent after methylnaloxonium injections into the locus coeruleus. Rearing and locomotor activity were also strongly increased after methylnaloxonium administration into the periaqueductal gray matter. Wet dog shakes were mainly observed after methylnaloxonium administration into the anterior preoptic hypothalamus and nucleus raphe magnus. Injections of methylnaloxonium into the amygdala produced a weak withdrawal syndrome, and the nucleus accumbens and medial thalamus were the least sensitive structures. These results suggest that the locus coeruleus, and secondarily the periaqueductal gray matter, play an important role in the precipitation of the physical signs of opiate withdrawal, mainly in the expression of its motor component. The expression of other signs of withdrawal appear to be multisite determined.