Nocturnal melatonin excretion was studied throughout a complete menstrual cycle in 10 women with migraine without aura attacks associated with menses and 9 women controls. Urine melatonin was determined by radioimmunoassay. The mean nocturnal melatonin excretion throughout the cycle was significantly lower in the migraine patients than in controls. In the control group, melatonin excretion increased significantly from the follicular to the luteal phase, whereas no difference was observed in the migraine group. Results are discussed in view of the role of the pineal gland in the organization of biological rhythms and homeostasis in relation to environmental conditions.