A pharmacokinetic study of fenquizone (Idrolone), a thiazide-like diuretic, was conducted with single oral doses in 6 healthy volunteers. The substance thus administered was readily absorbed from the gut, with peak plasma levels being detected on average at 3 h after dosing; after that, plasma concentrations of fenquizone decreased biexponentially in a pattern fitting an open two-compartment model. Plasma half-life values were 1 h for phase alpha and 17 h for phase beta. The half-life calculated from urinary concentrations was 18 h. The apparent distribution volume for phase beta was 686 l; renal clearance was 220 ml/min, and the absorption constant (Ka) was 1055 h-1. Cumulative urinary excretion accounted for 53.1% of the administered dose in 72 h. Thus the pharmacokinetic profile of fenquizone was that of an "intermediate-acting" diuretic about half-way between the short-acting hydrochlorothiazide, chlorothiazide and furosemide and the long-acting chlorthalidone. In summation, fenquizone is described as a low-dosage diuretic apparently not conducive to accumulation; its pharmacokinetic profile qualifies the product particularly well for maintenance therapy, such as is needed for the management of essential hypertension, both as sole medication and in fixed-ratio combination with beta-blockers, and at any rate with once-a-day administration.