Idiopathic Parkinson’s disease and dopaminergic medication may influence pituitary hormone secretion. The present study aimed to reveal any abnormalities of the somatotrophic system induced by the disease itself and/or the dopaminergic therapy. Investigations of other pituitary hormones under basal and stimulated conditions, as well as an analysis of body composition, were also performed. This was a controlled diagnostic study in which luteinising hormone-releasing hormone, thyroid-releasing hormone, corticotrophin-releasing hormone and arginine hydrochloride were administered to ten patients with idiopathic Parkinson’s disease under dopaminergic medication. Basal and stimulated hormone concentrations and bioelectrical impedance analyses were compared with those of healthy, age-matched controls. Basal growth hormone (GH) at −30 and 0 min was higher in Parkinsonian patients (2.74 ± 3.79 ng/ml versus 0.53 ± 0.10 ng/ml and 2.12 ± 2.44 ng/ml versus 0.51 ± 0.03 ng/ml; P < 0.05). The area under the GH curve after stimulation was greater in Parkinsonian patients (502.4 ± 202.6 ng × min/ml versus 312.0 ± 98.5 ng × min/ml; P < 0.05), depending on higher basal GH levels, rather than a greater arginine response. No differences in insulin growth factor (IGF)-1 or IGF-BP3 concentrations were detected. There were no differences between the groups in basal and stimulated gonadotrophic, corticotrophic and thyrotrophic function, or body composition. Prolactin was below the detection limit in the patients during the course of the study. Parkinsonian patients experience marked hypoprolactinaemia and repeated stimulation of GH secretion during chronic dopaminergic therapy. Our findings suggest a peripheral GH resistance in these chronically-treated patients.