Yohimbine is an alpha 2-adrenergic antagonist which has been proposed for the treatment of orthostatic hypotension (OH). We investigated in a double-blind placebo-controlled crossover trial the effects of yohimbine (2 mg tid) on blood pressure (BP) and heart rate (HR) using ambulatory monitoring of blood pressure (AMBP) in 17 patients (69 +/- 5 years) with Parkinson's disease (PD) suffering from non drug-induced OH. At the end of each sequence of 4 weeks, mean systolic (SBP) and diastolic (DBP) BP and HR (+/- SD), BP variability (coefficient of variation of the mean) and nychtemeral rhythm were calculated for day-time (09:00-19:00 h) and night-time (23:00-06:00 h). OH related to PD was characterized by: i) loss of the nychtemeral rhythm of BP (night/day ratio: 1.18 +/- 0.14) but not of HR (night/day ratio: 0.87 +/- 0.04), ii) high variability of BP during day-time and iii) post-prandial hypotension (12 out of 17 patients). There was no significant difference in AMBP parameters between baseline, yohimbine and placebo periods. This study shows that AMBP is a simple and reproducible method to assess the effects of drugs used in the management of OH on BP parameters (mean values and variability) and that yohimbine is not effective to correct OH of PD. Future studies must investigate other more selective alpha 2-adrenergic antagonists.