In a double-blind trial, the selective serotonin reuptake inhibitor citalopram was administered to women with severe irritability and/or depressed mood in the luteal but not in the follicular phase of the menstrual cycle (premenstrual dysphoria). Treatment continued for three consecutive menstrual cycles. One group (N = 17 completers) was administered citalopram continuously at a constant dosage (20+/-10 mg/day) throughout the menstrual cycle. A second group (N = 17) also received citalopram continuously throughout the cycle, but at a lower dosage in the follicular phases (5 mg/day) than in the luteal phases (20+/-10 mg/day) (semi-intermittent treatment). A third group (N = 18) received citalopram (20+/-10 mg/day) in the luteal phase only and placebo during the follicular phase (intermittent treatment). A fourth group (N = 17) received placebo throughout the cycles. The side effects of active treatment were generally mild and transient. Intermittent administration of citalopram was clearly more effective than placebo with respect to both reduction in self-rated irritability and self-rated global improvement; it is of interest that intermittent treatment with citalopram also seemed more effective than continuous or semi-intermittent administration of the drug.