A new study explores the interplay between the serotonin system and estradiol in the brain, showing that the central nervous system in patients with premenstrual dysphoric disorder (PMDD) seems to increase serotonin transporter density from the periovulatory phase (when estradiol levels are high) to premenstrual cycle phase (when both estradiol and progesterone are decreasing). The findings have the potential to advance the clinical treatment of PMDD.