It was determined by use of [35-S]furosemide and an ultrafiltration procedure that furosemide is bound extensively to bovine serum albumin. When 500 muM furosemide and albumin at a concentration of 2.5 g/100 ml were used, approximately 90% of the drug was bound. With this same amount of furosemide, but with 3 times as much albumin, binding was about 98%. Using a 25-fold lower concentration of furosemide, 20 muM, binding was nearly 98% with 2.5 g albumin/100 ml, and was over 98% with 7.5 g albumin/100 ml. These same concentrations of furosemide and albumin were used to investigate the excretory and secretory rates of [35-S]furosemide in the isolated perfused rat kidney. Tubular clearance (i.e., secretion) of [35-S]furosemide was inversely related to the concentration of albumin in the perfusate. In kidneys perfused without albumin, tubular clearance of the drug was 6-20 times that found when 2.5 or 7.5 g albumin/100 ml, respectively, was used. Probenecid, with or without albumin, reduced the clearance of furosemide to that of its filtration rate. It is concluded that at physiological albumin concentrations, a very small fraction of circulating furosemide will be available for filtration, and tubular-fluid and urinary furosemide will arise predominantly from secretion. Because of extensive binding of furosemide to albumin, the renal secretory process itself is depressed, and the rate of secretion will be dependent, in part, on the concentration of unbound drug.