The hypothesis that a dysfunction of serotonergic neurotransmission is implicated in depression is supported by the clinical efficiency of selective serotonin (5-hydroxytryptamine, 5-HT) reuptake inhibitors (SSRIs) in the treatment of depressive disorders. These drugs, such as fluoxetine and paroxetine, exert their antidepressant activity by increasing 5-HT concentration in the synaptic cleft and thus enhancing serotonergic neurotransmission. However, two to three weeks of treatment are necessary to see the first signs of clinical efficiency. Several hypothetical mechanisms have been put forward to account for this delay, taking into account pharmacokinetic considerations, neurotransmitter metabolism, and/or adaptive regulation of pre and/or post-synaptic receptors. The aim of this study was to look for such adaptive changes in the course of a 3-week treatment with fluoxetine (5 mg/kg/day, i.p.) or paroxetine (5 mg/kg/day, i.p.) in adult rats. In vitro binding and quantitative autoradiographic studies showed that neither 5-HT1A, 5-HT1B, 5-HT2A, nor 5-HT3 receptor binding sites in various brain areas were affected by these treatments. Furthermore, comparison of the specific binding of [3H]8-OH-DPAT to 5-HT1A receptors functionally coupled to G proteins with that of [3H]WAY 100635 to all 5-HT1A receptor binding sites (i.e. coupled and uncoupled with regard to G proteins) revealed no significant change in rats treated with either SSRI. Accordingly, the proportion of functional 5-HT1A receptors (i.e. those physically coupled to G proteins) appeared to remain unaltered all along a 3-week treatment with either fluoxetine or paroxetine. Nevertheless, in vitro electrophysiological recordings of serotonergic neurons in the dorsal raphe nucleus allowed the demonstration of a clearcut functional desensitization of somatodendritic 5-HT1A autoreceptors. Thus, the potency of the 5-HT1A autoreceptor agonist, 8-OH-DPAT, to depress the firing of serotonergic neurons in brain stem slices was significantly reduced as soon as after a 3-day treatment with either SSRI. The proportion of recorded neurons showing desensitization of somatodendritic 5-HT1A autoreceptors then increased along the treatment, and was generally larger with fluoxetine than with paroxetine. As 5-HT1A autoreceptor desensitization can contribute to facilitate serotoninergic neurotransmission, the remarkable efficiency of fluoxetine to trigger this adaptive regulatory mechanism might account, at least partly, for its potent antidepressant activity.