To investigate the pathogenesis of the normoprolactinemic galactorrhea syndrome, the response of prolactin secretion to TRH administration and the circadian profile of serum prolactin levels were examined in 13 women with galactorrhea whose resting levels of serum prolactin were lower than 25 ng/ml. Bromocriptine (5mg/day) was administered for 30 days and the mid-luteal serum estradiol and progesterone levels, as indicators of luteal function, were also measured before and after the administration. The basal levels of serum prolactin in these patients were significantly higher than those of the control (p less than 0.05), and the response of prolactin secretion also increased significantly at 30, 60, 90 and 120 min. after TRH administration compared to those of the control (p less than 0.005). The circadian profile of serum prolactin showed significantly higher levels from 22 to 8 o'clock compared to the control (p less than 0.05 approximately 0.005). And serum prolactin levels of these patients were higher than 25 ng/ml during the nocturnal period. When bromocriptine was administered, serum prolactin levels of these patients dropped conspicuously, and the nocturnal surges of prolactin also suppressed. Serum estradiol and progesterone levels in the mid-luteal phase normalized apparently due to the administration of bromocriptine (p less than 0.005, p less than 0.005), and galactorrhea also disappeared. These facts suggested that the normoprolactinemic galactorrhea syndrome might be caused by transient occulted hyperprolactinemia, and the treatment with bromocriptine was useful not only in suppressing galactorrhea but also in improving the luteal function in these patients.