Renal function studies have been performed before and during treatment with methyldopa in ten patients with different degree of renal functional impairment. Effective renal plasma flow increased during treatment in patients both with slight and advanced renal failure. However, ever, the glomerular filtration rate decreased slightly in several cases despite increased renal plasma flow. The renal hemodynamic effects of methyldopa could not be related to the serum concentrations of unconjugated methyldopa. The present study suggests that methyldopa is a useful drug for treatment of hypertension also in patients with impaired renal function. In patients with advanced renal failure the treatment should be given with caution because a further decrease of the glomerular filtration rate might occur.