Genetic deficiency of monoamine oxidase-A (MAO-A) induces major alterations of mood and behaviour in human. Because serotonin (5-HT) is involved in mood regulation, and MAO-A is responsible for the catabolism of 5-HT, we investigated 5-HT mechanisms in knock-out mice (2-month-old) lacking MAO-A, using microdialysis, electrophysiological, autoradiographic and molecular biology approaches. Compared to paired wild-type mice, basal extracellular 5-HT levels were increased in ventral hippocampus (+202%), frontal cortex (+96%) and dorsal raphe nucleus (DRN, +147%) of MAO-A mutant mice. Conversely, spontaneous firing rate of 5-HT neurons in the DRN (recorded under chloral hydrate anaesthesia) was approximately 40% lower in mutants. Acute 5-HT reuptake blockade by citalopram (0.2 and 0.8 mg/kg i.v.) produced a much larger increase in extracellular 5-HT levels (by approximately 4 fold) and decrease in DRN neuronal firing (with a approximately 4.5 fold decrease in the drug's ED50) in MAO-A knock-out mice, which expressed lower levels of the 5-HT transporter throughout the brain (-13 to -34% compared to wild-type levels). The potency of the 5-HT1A agonist 8-OH-DPAT to produce hypothermia and to reduce the firing of DRN serotoninergic neurons was significantly less in the mutants, indicating a desensitization of 5-HT1A autoreceptors. This was associated with a decreased autoradiographic labelling of these receptors (-27%) in the DRN. Altogether, these data indicate that, in MAO-A knock-out mice, the enhancement of extracellular 5-HT levels induces a down-regulation of the 5-HT transporter, and a desensitization of 5-HT1A autoreceptors which allows the maintenance of tonic activity of 5-HT neurons in the DRN.