Fluoxetine (10 mg kg-1, i.p.) caused a three- to fourfold increase in extracellular 5-hydroxytryptamine (5-HT) concentration measured by microdialysis in hypothalamus of freely moving rats. The addition of L-5-hydroxytryptophan at 20 or 40 mg kg-1, i.p. doses, magnified the increase in extracellular 5-HT to as much as 16 times basal levels, although these doses of L-5-hydroxytryptophan alone had only small effects on extracellular 5-HT. The increased formation of 5-HT following L-5-hydroxytryptophan administration appears to overcome homeostatic mechanisms that limit the increases in extracellular 5-HT caused by uptake inhibition.