Patients with premenstrual dysphoric disorder (PMDD) respond therapeutically to sleep deprivation and light therapy. They have blunted circadian rhythms of melatonin. The authors sought to test the hypothesis that these disturbances are a reflection of a disturbance in the underlying circadian pacemaker or, alternatively, that they reflect a disturbance in the input pathways to the clock. To test these hypotheses, after a 2-month diagnostic evaluation, 8 patients who met DSM-IV criteria for PMDD and 5 normal control (NC) subjects underwent two studies to determine whether PMDD subjects showed (1) altered melatonin sensitivity to light suppression (Study 1) and (2) altered phase-shift responses to morning light as a measure of the functional capacity of the underlying pacemaker (Study 2). In both studies, measurements were made during asymptomatic follicular and symptomatic luteal menstrual cycle phases in PMDD patients. The results of Study 1 showed no significant effect of group or menstrual cycle phase on the amount or percentage of suppression of melatonin by light. The results of Study 2 showed that with respect to the variable of offset time, PMDD subjects, when symptomatic, showed a reduced and directionally altered melatonin phase-shift response to a morning bright light stimulus; in 4 of 5 NC subjects, melatonin offset was advanced by bright morning light, whereas in PMDD subjects, it was delayed (3 subjects) or not shifted (5 subjects) (group effect, p = .045). Study 2 also revealed that area under the curve also changed differentially in PMDD versus NC subjects. In summary, the primary findings from this pilot study suggest that in PMDD there is a maladaptive (directionally altered and blunted) response to light in the symptomatic luteal phase. Because the suppressive effects of light were similar in PMDD and NC subjects, the previously observed low melatonin levels in this disorder do not likely represent a disturbance in pineal reactivity to suprachiasmatic nucleus efferents. Instead, the findings support a possible disturbance in PMDD in the clock itself or its coupling mechanisms.