Whereas serotonin reuptake inhibitors (SRIs) usually require weeks of treatment to effectively counteract depressed mood or anxiety, several studies suggest that they may reduce the symptoms irritability and anger more rapidly. In line with this, SRIs have been shown to reduce certain symptoms, including irritability, in women with premenstrual dysphoric disorder (PMDD) when the drug only is administered from ovulation to menstruation only, which indicates an onset of action of 10 days or less. How fast this effect appears in terms of hours or days is, however, not known. The objective of this study was to explore this issue.