The antiparkinsonian activity of bromocriptine, a presumed dopaminergic receptor agonist, was investigated in monkeys with surgically induced tremor and in a group of parkinsonian patients. A single administration of bromocriptine resulted in a dose-dependent relief of tremor in monkeys. Repeated administration enhanced this effect. Only mild abnormal involuntary movements were observed and only after repeated administration. Eleven patients with Parkinson's disease were treated with bromocriptine (mean dose, 26.4 mg a day). Clinically obvious improvement was noted in one or more of the cardinal signs of the disease in six patients (responders). No obvious improvement in any of the cardinal signs was noted in the remaining five patients (nonresponders). Clinically, the responders were older and more severely affected and had been on a higher dose of levodopa. However, they had had the disease for a shorter period. It is suggested that failure to respond to bromocriptine may be related to a decrease in the sensitivity of postsynaptic dopaminergic receptors.