Late Luteal Phase Dysphoric Disorder (LLPDD) refers to those women with Premenstrual Syndrome (PMS) who have predominantly affective symptoms. Evidence from recent small studies with well defined subject groups suggests that women with PMS/LLPDD may share several biological abnormalities with patients with major depression. These abnormalities include elevated nocturnal core body temperature, shortening of REM latency to a pathological level during the symptomatic period, diminished slow-wave sleep throughout the menstrual cycle, and decreased melatonin secretion. This application plans to simultaneously examine the sleep, core temperature and melatonin secretion of women with LLPDD and asymptomatic normal controls during two, 3 night sessions timed to coincide with the (asymptomatic) late follicular and (symptomatic) late luteal phases of the menstrual cycle. As pure as possible Cases of LLPDD will be identified by prospective daily ratings and careful psychiatric and medical screening. Sample sizes have been selected via power analyses to enable definitive statements regarding the hypotheses that: (l) women with LLPDD shorten their REM latencies when symptomatic as a state marker; (2) women with LLPDD show reduced slow-wave sleep and reduced power in the delta band of the sleep EEG throughout the menstrual cycle as a trait marker; (3) women with LLPDD have diminished melatonin secretion throughout the menstrual cycle as a trait marker; (4) women with LLPDD have elevated nocturnal core body temperature and temperature level is inversely related to melatonin levels in both LLPDD and normal control subjects. Confirmation that the above hypotheses are a part of the pathophysiology of LLPDD would (l) provide external validation of a disorder from which as many as 5 % of women of childbearing age suffer; (2) provide a basis for further research on the etiology and treatment of the disorder; and (3) provide preliminary evidence for the possible role of serotonergic dysregulation in LLPDD.