Bromocriptine, 2.5 mg twice a day was tested for its effect on premenstrual tension in a random double-blind cross-over trial. 1. The compound tended to lessen the symptoms, especially mastodynia. 2. Serum prolactin levels around the upper limit of the normal range were significantly lowered. Patients with the highest starting prolaction levels had the most severe symptoms and in these patients the fall in the levels was greatest. 3. Serum FSH and LH levels were significantly reciprocally influenced compared with serum prolactin. Serum estradiol-17-beta and progesterone did not change during treatment. 4. The bromocriptine-treated cycles were all ovulatory according to basal temperature levels. The luteal phase was prolonged when serum FSH was raised.