The kinetics of absorption of hydrochlorothiazide was studied in four adult male volunteers. Plasma levels were monitored for 48 h after a 100-mg dose. Food, electrolyte, and water intake were regulated as far as possible. The plasma level data were fitted to a two-compartment open model with either first-order or zero-order absorption, using the pharmacokinetic computer program AUTOAN. A better fit of the data, both graphically and statistically, was obtained with zero-order absorption kinetics. Additional confirmation of absorption kinetics was obtained by treating the plasma level results by the Loo-Riegelman method. Cartesian graphs of the amount absorbed verus time afforded linear plots, which indicate a zero-order process.