This study will be an exploratory, proof of concept, randomised, placebo-controlled, double-blind, multiple crossover study, with an open-label active run-in phase, in patients with multiple system atrophy (MSA) or Parkinson's disease (PD) who can concomitantly be treated with fludrocortisone and antiparkinsonian medication. Three sites in France and one site in Portugal will participate in this study.