The antiparkinsonian activity of bromocriptine and of lergotrile was investigated in monkeys with surgically induced tremor and in parkinsonian patients. Both drugs effectively relieve tremor in experimental monkeys and induce less pronounced abnormal involuntary movements than L-dopa or piribedil. Both drugs were shown to be of therapeutic value in a group of patients with advanced Parkinson’s disease who were no longer responsive to levodopa combined with carbidopa. Adverse effects were similar to those observed with levodopa and carbidopa, except that in individual patients abnormal involuntary movements and diurnal oscillations in performance (‘on-off effect) were decreased, while mental changes were increased. The interactions of bromocriptine and of lergotrile with dopamine and α-adrenergic receptors were investigated. Both drugs have mixed agonist-antagonist activities with respect to the dopamine receptors; lergotrile has a higher affinity for the agonist site while bromocriptine has a higher affinity for the antagonist site of the receptor. Both drugs effectively displace the binding of the α-adrenergic antagonist WB-4101 to cerebral cortex membranes. The mechanisms underlying the antiparkinsonian efficacies of these two drugs were discussed.