The relative contribution of serotonin (5HT) neurotransmission within the medulla (rostral ventromedial medulla) and forebrain (amygdaloid central nucleus and nucleus parafascicularis thalami) to the antinociceptive action of morphine microinjected into the ventrolateral periaqueductal gray (vPAG) was evaluated. The 5HT receptor antagonist methysergide was microinjected into the medulla, forebrain, (or both) after injection of morphine into the vPAG. The contribution of 5HT to the antinociceptive action of morphine was observed to depend on (a) the dose of morphine administered into the vPAG, (b) the site(s) at which methysergide was administered, and (c) the level of the neuraxis at which the behavioral assay was organized. Results of the present study were combined with those of previous studies from this laboratory and presented as a model of the mechanisms by which morphine administered into the vPAG generates its antinociceptive action.