The role of tubular accumulation in renal disposition and diuretic efficacy of hydrochlorothiazide was studied in the isolated perfused rat kidney. Hydrochlorothiazide resulted in a dose-dependent increase in the fractional excretion of sodium, chloride and potassium, and in urinary flow and pH. Renal clearance of the drug was low as a result of a low extraction ratio and extensive nonionic back-diffusion. Hydrochlorothiazide was subject to saturable tubular secretion, following Michaelis-Menten kinetics. Parameters obtained after nonlinear regression analysis were a maximum tubular transport velocity of 42 ± 6 µg/min, a Michaelis-Menten constant of secretion of 38 ± 11 µg/ml and a fraction of excreted drug reabsorbed passively of 0.49 ± 0.03. The thiazide diuretic accumulated extensively in kidney tissue due to active cellular uptake (maximum capacity of renal accumulation of 500 ± 270 µg/g; affinity constant of renal accumulation of 28 ± 16 µg/ml) and passive diffusion. Plots were constructed of the sodium excretion rate versus hydrochlorothiazide perfusate concentration or the renal excretion rate. The perfusate plot could be described by the sigmoid Emax model, while a simplification of the model had to be used for the response curve in urine because a maximum effect was not observed. The apparent maximum effect resulting from the perfusate concentration-response curve and the discrepancy with the renal excretion rate-response curve indicates that the diuretic effect of hydrochlorothiazide is restricted by saturable accumulation and s