4 Abnormality in the brain mechanism of error detection is thought to be one of the causes for the formation of obsessive-compulsive disorder (OCD). The key element of error detection mechanism is the anterior cingulate cortex (ACC). However, the data from neuroimaging studies on functional activity of the ACC at rest and when involved in maintaining brain functions are inconsistent. Previously we formulated the hypothesis that functional activity of ACC changes during the development of OCD. To test this hypothesis, a combined PET-fMRI study was conducted to investigate functional activity of the ACC in OCD patients at rest condition and during maintenance of the mechanism of error detection. To simulate such activity we used the modified GO/NOGO test with two tasks of different difficulty. In OCD patients at rest condition we revealed glucose hypermetabolism in the ACC, which decreases with increasing disease duration. Functional activity of the ACC during performance of GO/NOGO test was reduced in OCD patients compared to a group of healthy controls. Moreover, it was found that the activity of ACC increases with increasing disease duration during a more simple task but decreases during a more complex task. Thus, obtained data allow us to confirm the tested hypothesis. Glucose hypermetabolism in the ACC at the initial stages of the disease at rest condition is replaced by hypometabolism, probably because of the ACC dysfunction and compensatory reallocation of its functions among other relatively intact structures. This can lead to an abnormal functional specialization of ACC: Under high cognitive load conditions the ACC functional activity decreases with increasing disease duration while increases under relatively low cognitive load.