In healthy humans, there is an apparent dissociation between cognitive and affective consequences of reduced brain serotonin (5-HT), as rapid tryptophan depletion (RTD) causes alterations in a consistent constellation of cognitive processes in the general absence of mood deterioration. This study aimed to investigate possible neural mechanisms underlying this relative dissociation by measuring the effects of reduced 5-HT on regional cerebral blood flow (rCBF). A total of 16 healthy, euthymic male subjects (mean age 39±9 years) without a personal or family history of affective disorder had mood ratings and single photon emission computed tomography scans with the rCBF tracer 99m Tc-HMPAO under reduced 5-HT (RTD) and control conditions. Across individuals, modest positive and negative changes in subjective happiness associated with RTD were significantly correlated with change of rCBF in a cluster comprising subgenual (affective) anterior cingulate cortex (ACC) and associated regions (Brodmann's area (BA) 25, posterior BA11 and 47, caudate nucleus and ventral striatum; SPM99 p< 0.05, corrected). The covariation was such that increasing sadness was associated with increased rCBF and vice versa. Independent of mood change, RTD was associated with reduced rCBF in the dorsal (cognitive) ACC (BA32; SPM99 p< 0.05, corrected). The subgenual prefrontal cortex and dorsal ACC are important components of the ventral and dorsal neural systems that regulate and integrate affective and cognitive functions. The results therefore suggest that the dissociation between affective and cognitive consequences of RTD may possibly be attributable to differential effects of reduced 5-HT on these neural systems.