Fluctuations of total electrolyte concentration in the renal medulla were estimated from continuous measurement of tissue electrical admittance (reciprocal impedance) by means of needle electrodes placed in the kidney of anaesthetized rats. To compare effects of two diuretic agents with different sites of action, rats received either furosemide, 0.3 mg/kg i.v. followed by an infusion at 0.3 mg/kg.h, or acetazolamide, a single injection of 10 mg/kg. At this dosage similar increases in renal excretion were obtained with either drug. After furosemide (a loop diuretic) admittance fell sharp within first 10 min, then partly recovered and reached a plateau 35 min after injection. Acetazolamide (inhibitor of proximal reabsorption) caused no changes in admittance compared to the pattern observed in untreated control animals. We conclude that dissipation of tissue electrolytes from the renal medulla is not simply a consequence of diuresis and natriuresis but depends critically on the site of transport inhibition in the nephron.