The urinary protein binding, kinetics, and dynamics of furosemide were studied in five nephrotic patients after intravenous dosing. Serial plasma and urine samples containing furosemide were analyzed by HPLC, and drug binding to plasma and urinary proteins was determined using equilibrium dialysis techniques. In comparison to data reported previously in healthy subjects, the steady-state volumes of distribution and nonrenal plasma clearances were significantly increased in nephrotic patients, reflecting the reduced binding of furosemide to plasma proteins. Although there was no significant difference in renal clearance between these two groups, the unbound renal clearance of furosemide was significantly reduced in nephrotic patients even when compensated for by the number of functioning nephrons. Furosemide was extensively bound to urinary protein (19.6-78.4%), and the binding was dependent on the degree of proteinuria. Nevertheless, dose-response analyses, in which the response was represented by sodium excretion rate and the dose by urinary excretion rate of unbound drug, demonstrated that nephrotic patients were less responsive to equivalent amounts of unbound diuretic as compared to healthy subjects.