Ten patients with idiopathic Parkinson's disease (PD) (3 men and 7 women, group A) who had received no treatment for the disease; 102 patients with PD (36 men and 66 women, group B) who had undergone treatment and 45 healthy volunteers (15 men and 30 women, control group) were subject to thyrotropin-releasing hormone (TRH) tests and levodopa tests. In group A basal plasma prolactin (PRL) levels were significantly higher than in the controls both before and during treatment. Peak plasma PRL levels during TRH tests were significantly higher before treatment, but returned to the control levels during treatment. Nadir plasma PRL levels during levodopa tests were significantly increased before and during treatment. In group B basal plasma thyroid-stimulating hormone (TSH) and PRL levels were significantly higher than in the control group. Peak plasma PRL levels during TRH tests and nadir plasma PRL levels during levodopa tests were also significantly increased. The results strongly suggest a disturbance of pituitary hormone secretion due to hypothalamic dysfunction in PD patients.