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The response of plasma beta-endorphin (beta-EP) to dexamethasone suppression was studied in 14 patients with Alzheimer's disease (AD), 14 patients with Parkinson's disease (PD), and 13 age-matched controls in order to evaluate whether an impairment of the opiate system is present in these neurodegenerative disorders. Basal circulating beta-EP was in normal range in all subjects, although the mean concentration was slightly reduced in the patients compared to controls. After 1 mg dexamethasone given at 11:00p.m. the night before, plasma beta-EP concentration measured at 08:00 a.m. and 04:00 p.m. was not inhibited in AD and PD patients while it was significantly reduced in controls. Circulating ACTH and Cortisol were similar in patients and controls and a normal inhibition of plasma Cortisol after dexamethasone was observed in 13/14 AD and 12/14PD patients. The resistance of beta-EP to dexamethasone inhibition is consistent with previous clinical and experimental data indicating a disorder of the opiate system in brain degenerative diseases.