The pharmacokinetics of piretanide, a new loop diuretic, were studied in four patients with GFR 4.7-14.8 ml/min. An oral dose of piretanide 18 mg was given at 08.00 h in two patients and at 08.00 h and 14.00 h in two. Blood samples were drawn after 0.5, 1, 2, 3, 4, 6, 8, 10, 12 and 24 h. Serum concentrations of piretanide were estimated by radioimmunoassay. The peak serum concentration of piretanide (1-2 h after drug administration) was 510-880 ng/ml, independent of renal function. Elimination half life (t1/2) was 1.2-4.1 h, area under the curves (AUC(0,24)) 1.63-2.44 micrograms ml-1 h, volume of distribution (Vz) 0.30--0.741#kg, total plasma clearance (CL) 122.8-184.0 ml/min and renal clearance (CLR) 1.5-5.2 ml/min. The clinical effects of oral treatment with piretanide 18 mg twice daily were compared with bumetanide 3 mg twice daily in eight patients with renal failure (GFR 2.2-24.5 ml/min). Both drugs equally increased the 24 h output of urine (delta V), sodium (delta UNaV), chloride (delta UC1V), potassium (delta UKV) and calcium (delta UCaV). Fractional excretion of sodium (ENa%) was doubled by piretanide in patients with GFR less than 8 ml/min while a five fold increase was found in patients with GFR greater than 8 ml/min. The onset of effect was the same for both drugs, but the duration exceeded 6 h only for piretanide. Both drugs were most effective on the first of two consecutive treatment days. Delta UC1V was always greater than delta UNaV and urinary phosphate excretion was unchanged, as expected of a loop diuretic without significant proximal effects. Metabolic or clinical side effects were not noticed.