The neuroendocrinological changes that occur in patients with Premenstrual Dysphoric Disorder (PMDD) are poorly understood. The mechanism is likely to be a complex interaction between gonadal hormones and neurotransmitters. Melatonin is the predominant pineal neurohormone. The gonadal changes associated with melatonin are mediated via the hypothalamus. The purpose of this study was to evaluate melatonin concentrations in patients with PMDD. Also serum prolactin (PRL) and progesterone levels were estimated for any correlation with melatonin secretion. During the symptomatic late luteal phase, serum melatonin concentrations were determined by Radioimmunoassay (RIA) in 12 women with PMDD and 12 normal control women (NC). Also serum PRL and progesterone were estimated to all women by RIA. Melatonin concentration in patient with PMDD was significantly lower than in NC (mean 25.25 VS 79.25 P< 0.05). Serum PRL levels were significantly higher in PMDD subjects than in NC (mean 39.08 VS 20.17 P < 0.05). Serum progesterone levels were lower in PMDD than NC (mean 20.42 VS 33.58 P < 0.05). The results showed inverse correlation between melatonin and PRL levels (r = 0.931, P < 0.001) and direct correlation between melatonin and progesterone levels (r=0.894, p < 0.001). No correlation was found between PRL and progesterone level. Findings of the study suggest that lowered melatonin levels in women with PMDD may be due to involvement of the pineal gland in the pathogenesis of this disorder.