In 12 acromegalics a single oral dose of 0.2 mg lisuride, an ergoline derivative, significantly reduced plasma PRL but not GH concentrations. Three-tenths milligram of the drug significantly reduced plasma levels of the two hormones. Four-tenths milligram of lisuride did not augment this inhibitory effect. Plasma PRL levels were suppressed in all patients, whereas GH levels were reduced by more than 50% of the base-line values in only seven patients who also responded to the administration of 2.5 mg bromocriptine (CB154). In the patients unresponsive to lisuride, CB154 also failed to change GH levels. The suppressive effect of lisuride started at 60 min (at 150 min for CB154) and plasma GH and PRL levels were still markedly suppressed at 300 min. Plasma GH and PRL concentrations were consistently reduced in two acromegalic patients during 2 weeks of chronic treatment with 0.3 mg lisuride four times a day. In six normal subjects, TRH-induced PRL release was significantly inhibited by pretreatment with 0.3 mg of the drug. The similarity in the effects of lisuride and CB154 suggests that the observed effects of lisuride on GH and PRL are attributable to the known dopaminergic activity of the drug. This conclusion is supported by the data showing that pimozide effectively counteracted the inhibitory action of lisuride on GH and PRL release. We suggest that lisuride may be of value in the medical treatment of acromegaly and hyperprolactinemic states.