Tryptophan depleting protocols are commonly used to study the role of serotonin in mood disorders. The present study examined the impact of a tryptophan deficient diet and fluoxetine on the serotonergic regulation of neuroendocrine function and body weight. We hypothesized that the regulation of post-synaptic 5-HT_1A receptors is dependent on the levels of 5-HT in the synapse. Rats on a control or a tryptophan-deficient diet received daily injections of saline or fluoxetine (5 or 10 mg/kg/day i.p.) from day 7-21. The tryptophan-deficient diet produced a 41% reduction in the level of 5-HT but no change in the density of [³H]-paroxetine labeled 5-HT transporters. Treatment with fluoxetine inhibited the gain in weight in rats maintained on the control diet. The tryptophan deficient diet produced a significant loss in body weight which was not significantly altered by treatment with fluoxetine. Treatment with fluoxetine produced a dose-dependent desensitization of hormone responses to injection of the 5-HT_1A receptor agonist (±)8-OH-DPAT. The tryptophan-deficient diet produced an increase in the basal levels of corticosterone but did not alter the basal levels of ACTH or oxytocin. Also, this diet inhibited the magnitude of 8-OH-DPAT-induced increase in plasma levels of ACTH and oxytocin but did not impair the ability of fluoxetine to desensitize the 5-HT_1A receptor-mediated increase in plasma hormones. These data suggest that a reserve of 5-HT enables fluoxetine to desensitize post-synaptic 5-HT_1A receptors in the hypothalamus. Concluding, the profound physiological changes induced by tryptophan depletion may complicate the interpretation of studies using this experimental approach.