A significant body of evidence documented that the orbitofrontal cortex (OFC) and the head of caudate nucleus are involved in the mediation of obsessive-compulsive disorder (OCD) symptoms. Potent serotonin (5-HT) reuptake inhibitors (SRIs) are the only antidepressant agents thus far shown to be effective in the treatment of OCD. The present review summarizes information on 5-HT release and the adaptive changes in pre- and postsynaptic 5-HT receptors sensitivity induced by SRI treatment in rat and guinea pig structures involved in OCD. It emphasizes that the time course for the occurrence of increased 5-HT release and terminal 5-HT1D desensitization is congruent with the delayed therapeutic response to SRI in OCD. In addition, a greater dose of SRI inducing a greater degree of reuptake inhibition may play an essential role in this phenomenon. This is consistent with the common clinical observation that high doses of SRIs are sometimes necessary to obtain an anti-OCD effect, and with the results of some fixed-dose double blind trials showing a dose-dependent therapeutic effect of SRIs. It is hypothesized that enhanced 5-HT release in the OFC is mediated by the activation of normosensitive postsynaptic 5-HT2-like receptors and underlies the therapeutic action of SRI in OCD. This is supported by the beneficial effect of some hallucinogens with 5-HT2 agonistic properties in obtaining a more rapid therapeutic response. Finally, based on this knowledge, new strategies aimed at producing more rapid, effective and safe anti-OCD drugs, such as a selective action on terminal 5-HT1D receptors, on 5-HT2 receptors as well as on the glutamate system, are discussed.