To assess the action of piretanide, bumetanide, and furosemide on the thick ascending limb of Henle's loop, perfusion experiments were performed on rat kidneys, using a recently developed conductivity microprobe for the analysis of distal tubular fluid. Surface nephrons were perfused downstream from a solid paraffin block through Henle's loop with isotonic saline solution containing either 10(-6), 3 X 10(-6), or 10(-5) mol/L piretanide, bumetanide, or furosemide, respectively. For control values, diuretic-free saline was used. Conductivity measurements of early distal tubular fluid were performed in situ with a conductivity microprobe and were recorded continuously during variations of loop perfusion rate. A recently described micro-stop-flow technique was used to measure the conductivity of fluid emerging from the ascending limb, thereby evaluating the amount of sodium chloride reabsorbed. A significant electrolyte transport inhibition occurred within the concentration range between 10(-6) and 10(-4) M. Bumetanide and piretanide were slightly more effective than furosemide. Luminal diuretic concentrations of 10(-6) M or below were without influence on electrolyte reabsorption. Concentrations of 10(-4) M resulted in complete inhibition of the diluting segment electrolyte transfer.