Potent serotonin (5-HT) reuptake inhibitors are the only antidepressant agents thus far shown to be effective in the treatment of obsessive-compulsive disorder (OCD). Positron emission tomography studies in humans have implicated the orbito-frontal cortex and the head of caudate nucleus in the mediation of OCD symptoms. Since the delay of the maximal therapeutic effect of selective 5-HT reuptake inhibitors (SSRI) is longer in OCD than in major depression and the terminal 5-HT autoreceptor is not desensitized in the guinea pig frontal cortex after 3 weeks of SSRI administration, the effects of the SSRI paroxetine (10 mg/kg/day) and fluoxetine (5 mg/kg/day) on 5-HT release and on the sensitivity of the terminal 5-HT autoreceptor were investigated in the guinea pig frontal cortex, the orbito-frontal cortex, and the head of caudate nucleus following a washout period after 3 and 8 weeks of treatment. In preloaded slices prepared from guinea pigs treated with paroxetine for 3 weeks, the electrically evoked release of [3H]5-HT release was enhanced in the frontal cortex (21%) but not in the orbito-frontal cortex or in the head of caudate nucleus. However, after an 8-week treatment, the evoked release of [3H]5-HT was significantly enhanced in the orbito-frontal cortex (55%) and in the rest of the frontal cortex (29%) from the same animals, but still unchanged in the head of caudate nucleus. Concentration-effect curves, constructed with the 5-HT autoreceptor agonist 5-methoxytryptamine, showed that the terminal 5-HT autoreceptor was desensitized only in the orbito-frontal cortex after 8 weeks of treatment with paroxetine. Furthermore, the 5-HT transporter was desensitized in the frontal cortex but not in the orbito-frontal cortex. In the case of 3- or 8-week fluoxetine treatment, neither [3H]5-HT release nor the sensitivity of the terminal 5-HT autoreceptor were altered in the orbito-frontal cortex and the head of caudate nucleus. This could be attributable to a smaller degree of 5-HT reuptake inhibition achieved with fluoxetine, in keeping with the notion that higher doses of SSRI are generally required to improve OCD than depression. Taken together, these results indicate that, in the orbito-frontal cortex, the enhanced release of [3H]5-HT induced by prolonged and marked 5-HT reuptake inhibition is attributable to a desensitization of the terminal 5-HT autoreceptor.