Increases in anxiety levels during the late-luteal phase of the menstrual cycle form important diagnostic criteria of premenstrual dysphoric disorder (PMDD) (1). Evidence exists to support the hypothesis that tolerance to endogenous levels of melatonin might occur during the luteal phase in PMDD (2, 3). It was hypothesized that slow release (SR) melatonin administration during the luteal phase of the menstrual cycle of participants with PMDD could significantly lower anxiety levels measured by self-report
