It is known that the serotonin reuptake inhibitor escitalopram is effective for the treatment of premenstrual dysphoric disorder. The aim of the present study is to investigate the mechanisms underlying this effect. The questions to be addressed are i) if women with PMDD display differences in blod and/or CSF between the symptomatic (premenstrual) and the asymptomatic (postmenstrual) cyclic phase regarding levels of gonadal steroids, neurosteroids, inflammatory markers or various markers of brain neurotransmission, ii) to which extent these parameters correlate with each other and/or symptom intensity and iii) to what extent these parameters are affected by treatment with escitalopram.