The effect of the thiazide diuretic, bemetizide, on the excretion of Na+, K+, Cl-, Ca2+, and Mg2+ in relation to the glomerular filtration rate (GFR) was studied in 17 subjects whose creatinine clearances ranged from 133 to 5 ml.min-1. After a 2-day fluid and salt balanced control period, 25 mg bemetizide given orally induced natriuresis and kaliuresis which lasted for 24 h and were proportional to the GFR of the patients. The ratio of bemetizide-induced K+/Na+ excretion was always 0.17 irrespective of individual GFR. In renal failure, bemetizide increased the fractional Na+ excretion from 3% to about 10%. Kaliuresis was associated with magnesiuria, whereas bemetizide-induced calciuresis was insignificant. The thiazide reversibly lowered GFR in all subjects.