18 hypertensive patients with a glomerular filtration rate (GFR) between 3.8 and 113ml/min received guanfacine as single intravenous and multiple oral dose treatment. Mean plasma concentrations of guanfacine on the fifth day of oral treatment ranged from 6.5 to 8.6ng/ml in patients with normal renal function (GFR > 90ml/min), as well as in those with a moderate degree of renal impairment (GFR 30 to 10ml/min) and in uraemic patients (GFR <10ml/min). During guanfacine treatment plasma concentrations and fall in blood pressure did not differ in a statistically significant fashion among patients with varying degrees of renal function.Independent of renal function, in all investigated patients the elimination rate constant obtained from serum and urine values were close to 0.05h-1. Mean half-life of guanfacine was calculated to be 14h. The cumulative urinary excretion up to 48h fell from 57% in patients with normal renal function to 7.5% in uraemic patients. Thus, non-renal excretion plays an important role in patients with impaired renal function.