To investigate adaptative changes of 5-HT2A receptors induced by SSRIs, six patients chronically treated for a depressive episode (four with fluoxetine, two with fluvoxamine) were studied with PET and [18F]setoperone. They were compared to eight untreated depressive patients. The mean frontal to cerebellum radioactivity concentration ratio, an index of the [18F]setoperone specific binding to 5-HT2A receptors, was higher in treated than in untreated patients, when age was taken into account. This suggests that chronic treatment by SSRIs could induce an up-regulation of the 5-HT2A receptors, and that 5-HT2A receptor down-regulation is not a common mechanism for the therapeutic effects of all serotoninergic antidepressive drugs.