The effects of pregnancy, delivery and lactation on changes in serum prolactin (PRL) values were investigated in patients with hyperprolactinemia. Thirty-seven patients with hyperprolactinemia who wished to become pregnant were treated by transsphenoidal surgery, bromocriptine therapy, or a combination of the two. In 33 patients whose pre-pregnancy serum PRL concentration exceeded 30ng/ml, only in two did serum PRL return to the normal range below 30ng/ml after pregnancy, delivery and lactation. However, the serum PRL concentration was decreased in 28 patients. When classified according to the pre-pregnancy serum PRL concentrations, PRL less than or equal to 100 (Group A), 100 less than PRL less than or equal to 200 (Group B) and 200 less than PRL (Group C), patients with the greatest pre-pregnancy serum PRL concentration showed the greatest reduction. The ratios of post-pregnancy serum PRL to pre-pregnancy PRL in group A, B and C were 91.4 +/- 22.1%, 81.5 +/- 7.0% and 65.0 +/- 6.5% (Mean +/- SE), respectively. Group C with the highest pre-pregnancy serum PRL concentration consisted almost entirely of patients with macroadenoma. Thus, the reduction in serum PRL after pregnancy, delivery and lactation was considered to be the result of a decrease in the size of the adenoma due to adenoma enlargement over the sella turcica through the estrogen effects during pregnancy, and from impairment of pituitary circulation.