In order to evaluate whether the stimulating effect of GABA on growth hormone (GH) secretion changes in patients affected by Parkinson's disease, ten male parkinsonian patients and ten age matched normal controls were tested with the GABA derivative and GABAergic agent Baclofen (10 mg in a single oral administration at 09.00 h) (experimental test). In a different occasion, normal men and parkinsonian patients were tested with a placebo (control test). Basal GH levels were similar in normal controls and parkinsonian patients and remained unmodified during the control test. Plasma GH levels rose three times within 120 min after the administration of baclofen in the normal subjects. In contrast, plasma GH concentrations remained unmodified after baclofen treatment in the parkinsonian patients. In agreement with previous reports in the literature showing alterations of GABAergic neurotransmission in the parkinsonian brain, these data show a reduced GABAergic control of GH secretion in patients with Parkinson's disease.