Levodopa (LD) application improves motor symptoms in patients with Parkinson's disease (PD) patients. Little is known on further effects of LD, which induced lower plasma levels of cortisol and lower serotonergic activity in certain brain areas of fish. Objectives of this trial were to analyse levels of cortisol, LD and 3-O-methyldopa (3-OMD) after administration of LD/benserazide in long term treated PD patients. 12 PD patients, taken off their regular treatment for at least 12 hours, received soluble 200 mg LD/50 mg benserazide under stress free conditions. Motor symptoms improved, LD and 3-OMD levels increased, whereas cortisol concentrations started to decrease significantly 30 minutes after LD intake. This reduced cortisol release may result from an overflow of exogenous LD in the brainstem. This hypothetically causes an reduced 5-HT content in neurons projecting to the hypothalamic structures, which are involved in the partial 5-HT dependent central regulation of peripheral cortisol release.