In order to establish whether the serotonergic disorder affecting the parkinsonian brain also modifies hypothalamic-pituitary function in Parkinson’s disease, 10 patients (aged 57–66 years) and 10 normal controls (aged 55–69 years) were tested with the serotonergic agonist d,l-fenfluramine (60 mg p.o.), with CRH (100 µg i.v.) and with placebos. Plasma ACTH/cortisol levels during tests were evaluated and compared. Both groups showed similar levels of ACTH and cortisol in basal conditions and after placebo administration. A slight physiological decline in both ACTH and cortisol levels during the placebo test was observed in normal controls and parkinsonian patients. CRH induced similar ACTH/cortisol increments in all subjects. In contrast, d,l-fenfluramine significantly increased plasma ACTH/cortisol concentrations in the normal controls, but not in the parkinsonian patients. These data show a defective serotonergic control of the pituitary-adrenal axis in Parkinson’s