To investigate further the role of growth hormone (GH) on the cerebral effects of levodopa, human somatotropin (human growth hormone) was administered to four patients with parkinsonism with varying response to chronic levodopa therapy. The doses of somatotropin (5 IU) were administered on alternate days for four days, and the effects of this hormone on the patient's symptomatic control, dyskinesia, plasma GH, and dopa levels were evaluated and compared with those of patients receiving saline injections. We have demonstrated that administration of exogenous human somatotropin, even during marked and sustained elevations of plasma GH levels, does not alter the therapeutic or side effects of levodopa therapy. With one exception, plasma dopa levels after human somatotropin administration remained unchanged. We conclude that neither the endogenous increases of GH in response to levodopa nor those attained following human somatotropin administration modify in any consistent way the therapeutic and side effects of chronic levodopa therapy, and that the episodic releases of GH in response to levodopa occur independently of its cerebral effects.