Tizolemide, an alkaline sulphonamide diuretic, was given i.v. in one dose and orally during one week to eight patients with compensated cardiac failure. They had essentially normal glomerular filtration rate but reduced renal plasma flow. Tizolemide was almost completely absorbed from the gastrointestinal tract. The drug was mainly eliminated via tubular secretion. Renal clearance of the drug was much lower than in healthy subjects because of low renal plasma flow. As a consequence plasma half-life was prolonged considerably in some patients. It was concluded that drugs with mainly tubular renal elimination may have a reduced elimination rate in patients with cardiac diseases despite normal glomerular filtration rate.