The aim of the present study was to determine whether the previously observed desensitization of hypothalamic 5-hydroxytryptamine1A (5-HT1A) receptors, during daily injections of fluoxetine, is mediated by sustained blockade of 5-HT reuptake. In the present study, we examined the time course effects of another 5-HT uptake inhibitor, paroxetine. Paroxetine reduced the oxytocin, adrenal corticotropic hormone and corticosterone responses to a challenge with the 5-HT1A agonist 8-hydroxy-2-(dipropylamino)tetralin. These reductions in hormone responses were significant after 3 daily injections and reached a maximum after 7 daily paroxetine injections. These hormone responses remained maximally suppressed after 14 daily injections of paroxetine. A single day of paroxetine treatment did not alter the hormone responses to 8-hydroxy-2-(dipropylamino)tetralin. Repeated injections of paroxetine did not reduce the density of 5-HT1A receptors in any brain region but did produce a gradual reduction in the levels of G(i) and G(o) proteins in a region-specific manner. The time course of the paroxetine-induced reduction in the level of G(i1) and G(i3) proteins in the hypothalamus was similar to the effect previously observed with fluoxetine and was also similar to the time course of paroxetine-induced reductions in oxytocin and adrenal corticotropic hormone responses to 8-hydroxy-2-(dipropylamino)tetralin. In conclusion, these results suggest that blockade of 5-HT uptake sites produces a delayed and gradual desensitization of 5-HT1A receptors in the hypothalamus. This desensitization is not due to changes in the density of hypothalamic 5-HT1A receptors. Reduction in the hypothalamic level of G(i3) proteins may play a role in the desensitization of 5-HT1A receptor systems. However, reductions in G(i1) or G(o) proteins cannot be excluded as potential mediators of the desensitization of 5-HT1A receptor systems.