We have previously suggested that albumin in tubule fluid at concentrations found in the nephrotic syndrome (NS) binds furosemide, thereby diminishing diuretic effect. This mechanism may contribute to diuretic resistance in NS. If this hypothesis is correct, displacement of albumin from furosemide should restore diuretic response in tubule fluid containing albumin. To test this supposition, in vivo loop microperfusion was performed in rats using perfusates containing 6 microM furosemide in the presence or absence of 3.8 microM albumin, or furosemide and albumin to which 12 microM warfarin or 5.4 mM sulfisoxazole had been added. These drugs are inhibitors of albumin-furosemide binding in plasma. Albumin in the perfusate impaired furosemide effect on loop chloride reabsorption (1248 +/- 59 vs. 886 +/- 65 pEq/min; P less than 0.05). Addition of warfarin or sulfisoxazole to perfusate containing albumin normalized furosemide's effect. Neither drug affected furosemide response in the absence of albumin. Dansylsarcosine, a probe that binds albumin at a different site than furosemide, failed to normalize furosemide response in albumin perfusates. These data suggest that albumin in tubule fluid reduces diuretic response through a diminution in the free furosemide concentration. In as much as this mechanism contributes to diuretic resistance observed clinically in NS, displacement of furosemide from albumin binding sites may be a therapeutic strategy warranting study.