Ethinylestradiol (400 µg/day) does not only stimulate prolactin release in normal cycling women but also modifies the pattern of the circadian periodicity of circulating prolactin: the nocturnal rise is of reduced amplitude but covers a larger part of the nyctohemeral period as compared to the situation during a control period in the same subjects. The values start rising long before sleep, i.e., at 2 PM, under estrogen treatment and during sleep, i.e., at 4 AM, during the control period. In both cases, however, the values remain high until 8 AM and are minimal at 12 AM. (J Clin Endocrinol Metab40: 886, 1975)