Prolactinomas are one of the most frequent tumors of the human anterior pituitary. Dopamine agonists are the choice in the medical treatment of this disease. Bromocriptine (BC) is a well known anti-neoplasic agent in human PRL secreting adenomas although its effect on PRL cells is far from clear. We decided to investigate its influence on cell proliferation parameters: (3H)thymidine incorporation, expression of c-myc and c-fos, and number of estrogen receptors present in the samples. A total of 28 patients were included in this protocol. They were treated with BC (5-7.5mg day(-1) patient(-1)) or with vehicle orally 15 days before surgery. We found that in BC treated patients (3H)thymidine incorporation was lower than in vehicle treated patients. The oncogenes expression were diminished in BC comparing with vehicle-treated patients. No difference in the number of estrogen receptors was observed in the samples from BC or vehicle-treated patients. These results clearly demonstrate that one mechanism to reduce the size of human PRL secreting adenomas by BC is the inhibition of DNA duplication.