Magnetic resonance imaging (MRI) with a superconductive 0.5 T magnet has been used in 10 patients with pituitary adenomas to evaluate the effect of bromocriptine treatment. Seven patients had prolactinomas, one a growth hormone (GH) producing adenoma and two patients null cell adenomas with slight hyperprolactinaemia. A single dose of 50 mg bromocriptine was given intramuscularly in a slow release preparation, followed after 6 weeks by 5 mg/day orally. Significant reduction of serum prolactin was noticed in all patients. Visual field defects disappeared rapidly. In all prolactinoma patients and in the patient with the GH producing adenoma MRI showed a decrease in tumour size. In 5 of these 8 patients MR signal analysis, aided by the use of a subtraction technique, clearly visualized the development of large intratumour cysts. In one further patient with a null cell adenoma there was also a rapid development of an intratumour cyst and an increase in tumour size. This patient was treated with transsphenoidal needle aspiration. In all tumour cases an increase in intensity quotients based on T1 and T2 relaxation times was calculated.