Migraine has been considered a manifestation of sympathetic dysfunction. Serum dopamine beta-hydroxylase (D beta H) has been shown to be an index of peripheral sympathetic activity by some workers and there are two reports of elevated activity of the enzyme during the migraine headache as well as in the headache-free interval. We studied the enzyme in seven women complaining of regular attacks of menstrual migraine and eleven controls during the mid-follicular (days 10 +/- 2) and premenstrual (days 28 +/- 2) phases of the menstrual cycle. Although levels were on average 26% and 10% higher respectively than in control subjects, the difference failed to reach statistical significance because of the large normal range for enzyme activity. However, the premenstrual results were significantly lower (p less than 0.001) than the mid-follicular measurements in the migraine group, little difference being found in controls. This finding, and the effects of successful therapy with anovulatory doses of oestradiol implants in not only significantly lowering serum D beta H but also significantly reducing the difference in enzymic activity between the early and late phases of the menstrual cycle, suggest that if this enzyme is an index of sympathetic activity, it is excessive fluctuations of the sympathetic nervous system that may be relevant in menstrual migraine.