The purpose of this study was to investigate modifications of 5-HT synthesis in a chronic pain model, the arthritic rat, at different times after the inoculation with Freund's adjuvant. This study confirms our previous findings that experimental induced polyarthritis is associated with a marked increase in free tryptophan levels in serum. During the acute phase of the disease (15-21 days after the adjuvant), the general increase in 5-HT synthesis observed in the CNS appeared to be related to an increase in tryptophan availability due to the elevation of free tryptophan in serum. During the post-acute phase of the disease (28-42 days after the adjuvant), the level of free tryptophan in the serum remained markedly increased but the levels in the CNS tended to return to normal values in all areas examined. At 42 days, 5-HT synthesis in the brain had also returned to normal values but was further increased at the spinal level. In addition, although 5-HT levels and 5-HT synthesis were increased in the dorsal as well as in the ventral part of the cord, an increase in the rate of disappearance of the amine after blockade of the decarboxylase (benserazide) was only observed in the dorsal part. This result tends to suggest that the descending serotonergic system projecting to the dorsal horn is preferentially activated during chronic pain.