Sex hormones such as estradiol and testosterone modulate human brain structure and function and are tightly connected to neuropsychiatric disorders such as depression and anxiety disorders. Using molecular imaging in humans in vivo, the investigators showed strong influences of sex hormones on serotonergic neurotransmission via modulation of serotonergic receptors and transporters. Although, animal studies also indicate strong modulatory influences on serotonin synthesis and degradation, human data on this potential effect are absent.