Since prolactin has several modes of action on prostatic growth and physiology, the effect of the antiprolactin bromocriptine on plasma kinetics and intraprostatic metabolism of testosterone was studied in patients with untreated prostatic cancer; a therapy protocol was deduced which was controlled in 27 patients with advanced inoperable prostatic adenocarcinoma. Bromocriptine resulted in a significant suppression of prolactin and testosterone as well and favored testosterone elimination from the plasma pool. Prostatic androgen uptake was enhanced and the intraprostatic metabolism altered in relation to tumor grade. Adjunctive administration of bromocriptine to 27 patients, mostly in the state of hormone resistance, resulted in an overall objective regression of 22.2% and in stable disease in 55.6% of the patients. In half of the individuals a prompt relief of bone pain from osseous metastases was observed as well as improvement of micturition and decline of phosphatase activity. This preliminary data justify further investigations under controlled and randomized conditions.