Twelve acromegalic patients were studied in one or more of three protocols to establish the response of plasma growth hormone (hGH) concentraions to (a) a single oral test dose of bromocriptine, (b) incremental dose therapy from 10 mg/day to 40 mg/day over 4 weeks, and (c) sustained therapy with 20 mg/day over a 3 month period. Ten of the patients studied had previously been treated by yttrium implantation, external pituitary irradiation or surgical hypophysectomy. A high incidence of side effects including postural hypotension, hallucinations and peripheral vasospasm was noted reducing the numbers of patients completing the three protocols. Suppression of hGH concentrations was disappointing; significant reductions in hGH concentraions occurred in five of eleven patients in response to a single test dose of bromocriptine, and in only one of seven patients treated with the drug over a sustained period. An impression of clinical improvement was gained in one patient, but there was no associated reduction in plasma hGH concentrations. It seems likely that acromegalic patients who have failed to respond to conventional treatment may be more resistant to bromocriptine therapy.