Although anovulation associated with hyperprolactinemia is not an uncommon cause of infertility, the precise mechanism of the pathogenic process that induces hyperactivity (hypertrophy with hyperplasia) of pituitary lactotropes is unknown. We have recently experienced a case of anovulation and hyperprolactinemia in a woman with ergot alkaloid intolerance in whom ovulation was restored by tamoxifen citrate administration. Since tamoxifen citrate administration also suppressed prolactin levels, it was suggested that a low but sustained serum level of estradiol and consequently continuous estrogenic stimulation may be an important causative factor in the development of hyperprolactinemic anovulation.