The new long-acting ergoline derivative cabergoline was given orally in a single dose of 300 micrograms to 15 hyperprolactinemic patients (including 4 acromegalic patients, 2 of whom were dopamine responsive). Serum PRL and GH levels were determined before and 3, 4, and 6 h and 1, 2, 3, 4, 5, 6, and 7 days after treatment. A control test with a single oral dose of 2.5 mg bromocriptine was also performed; serum PRL and GH levels were measured at the same time intervals for 2 days. Cabergoline induced a marked fall in serum PRL which began within 3 h and continued for 7 days. The maximal decrease ranged between -49.2% and -55.2% and occurred after 2-5 days. This maximal effect was only slightly less than that 6 h after bromocriptine treatment (-63.8%). After cabergoline treatment, serum GH levels did not change significantly in either nonacromegalic or acromegalic patients, whereas the two dopamine-responsive acromegalic patients had a marked GH fall after bromocriptine. A moderate blood pressure decrease, more evident in the standing position, occurred after both cabergoline and bromocriptine treatments. The only symptomatic side-effect was orthostatic hypotension after cabergoline in an elderly woman. These data indicate that cabergoline has potent and prolonged dopaminergic activity and may prove suitable for once weekly treatment of hyperprolactinemic patients.