Fusaric acid (FA) administration to the rats promoted one hour later a large decrease in plasma total tryptophan (TRP), without affecting either plasma free TRP or lipolysis, as measured by plasma non esterified fatty acid concentration. The previous change was associated with hypoinsulinemia, hyperglycemia and increased plasma corticosterone level. Regression analysis revealed a significant correlation between brain TRP and the percentage of plasma TRP which was free (i.e. unbound to albumin), both increased by FA injection. The increase in brain TRP promoted an increased brain serotonin synthesis, as measured by the enhanced brain and CSF 5-HIAA levels. Valine pretreatment, which blocks TRP entry into the brain, completely prevented FA-induced brain TRP and brain 5-HIAA increases. These results suggest that the increased brain serotonergic turnover following FA treatment was due to a peripheral action of the drug upon TRP disposition. The latter effect may be caused (i) by in vivo peripheral alterations in catecholaminergic metabolism and (ii) by FA chemical structure since in vitro experiments revealed that FA was able to displace TRP binding to albumin, thus increasing the plasma free TRP pool.