Parkinson's disease (PD) is an age-related neurodegenerative disease characterized by a progressive motor disorder, but frequently is accompanied by autonomic symptoms such as hypotension. Together with the decrease of dopamine, significant decreases in aminopeptidase activities have been reported in PD brains. However, up to date there are no studies about changes of aminopeptidase activities in plasma of PD patients. We studied plasma activities of alanyl-, aspartyl-(AspAP), cystinyl-(CysAP) and glutamyl-aminopeptidase (GluAP) in two groups of subjects: control (n=41) and PD (n=48). Plasma activities of AspAP, CysAP, and GluAP showed significant decreases of 24.9% (p<0.05), 39.4% (p<0.01) and 33.3% (p<0.01), respectively, in PD group. These aminopeptidases are involved in the metabolism of circulating peptides such as the ones of the renin-angiotensin system. The importance of aminopeptidases in striatal dopamine content and in neuroendocrine system in PD is discussed.