The time course of fluoxetine-induced desensitization of hypothalamic 5-hydroxytryptamine1A receptors was examined in rats. Daily injections of fluoxetine (10 mg/kg/day) for 0, 3, 7, 14 or 22 days gradually produced a shift to the right in the dose-response curve effects of 8-hydroxy-2-(dipropylamino)tetralin (8-OH-DPAT) on plasma adrenal corticotropic hormone, corticosterone and oxytocin. A partial reduction was observed for the adrenal corticotropic hormone and oxytocin responses to 8-OH-DPAT (50 micrograms/kg s.c.) after 3 days, and a maximum reduction of all hormone responses was observed after 14 days of fluoxetine injections, when the adrenal corticotropic hormone and oxytocin responses to the 50-micrograms/kg dose of 8-OH-DPAT were virtually blocked. To begin to examine the mechanism of 5-hydroxytryptamine1A receptor desensitization, we determined levels of Gi and G(o) proteins in the hypothalamus, midbrain and frontal cortex by using immunoblots. The hypothalamic levels of Gi1 and Gi3 proteins were significantly reduced after 7 and 14 days of fluoxetine injections. The levels of G(o) and Gi2 proteins in the midbrain were significantly decreased after 3 days and remained reduced for the duration of fluoxetine injections. Fluoxetine did not reduce the concentrations of Gi and G(o) proteins in the frontal cortex at any time. The similarity in time course between fluoxetine-induced reductions in hormone responses to 8-OH-DPAT and the reduction in hypothalamic levels of Gi1 and Gi3 proteins suggests that a reduction in hypothalamic levels of Gi3 and/or Gi1 proteins plays a role in the gradual desensitization of 5-hydroxytryptamine1A receptors induced by fluoxetine.