Eight patients with terminal renal failure (GFR less than 5 ml/min) received guanfacine intravenously or orally at the beginning of a haemodialysis session lasting 5 hours. After intravenous administration the elimination rate constant of the unchanged drug was calculated to be 0.59 h-1, corresponding to an elimination half-life of about 12 h, which is similar to the values of patients with normal renal function. Following oral ingestion, only 2.4% of the guanfacine dose administered were extracted unchanged in the dialysate during the haemodialysis period. The clearance by dialysis was 53 ml/min for guanfacine, which represents about 15% of the total clearance of guanfacine reported in the literature. The low dialysis clearance of guanfacine is consistent with the high rate of biotransformation of the drug and its binding to red blood cells and plasma proteins. Thus guanfacine may be dosed independently of haemodialysis treatment periods.