The therapeutic and adverse effects of two ergot derivatives, bromocriptine and lergotrile, were compared in idiopathic parkinsonism. At both low (50 mg daily) and high (150 mg daily) dosage there was a similar but not identical profile of response. Initially, lergotrile tended to induce more severe but always transient hypotension. At higher doses, bromocriptine caused more dyskinesia. Neurological deficits improved with increasing doses up to an average daily level of 80 to 150 mg of ergot derivatives combined with levodopa, 450 to 1,150 mg, and carbidopa, 45 to 115 mg. However, efficacy often declined at the highest doses of antiparkinsonian agents. Adverse effects caused by ergot derivatives are more common with dosages greater than 100 mg per day. In general, the best overall therapeutic results with bromocriptine and lergotrile were obtained in the dose range of 50 to 100 mg daily for each. It is concluded that bromocriptine and lergotrile are similar in their therapeutic properties and that both are comparable in efficacy to levodopa plus carbidopa (though optimal results are commonly obtained by combining submaximal doses of levodopa with ergot derivatives). The role for each drug in the treatment of parkinsonism is likely to be determined by factors such as cost (bromocriptine) and hepatotoxicity (lergotrile).