In this study we compared the absorption and disposition of two commonly used combination formulations of hydrochlorothiazide and triamterene (Dyazide and Maxzide) in 48 patients with essential hypertension after dosing with each formulation to steady state. Interdose AUC and urinary recovery of hydrochlorothiazide, triamterene, and the major metabolite of triamterene, hydroxytriamterene sulfate (adjusted for dose), documented marked impairment in the absorption of hydrochlorothiazide (approximately two third as bioavailable) and triamterene (about half as bioavailable) from Dyazide in comparison to Maxzide. The study also demonstrated a reduction in the clearance of triamterene, hydrochlorothiazide, and hydroxytriamterene sulfate with increasing age. Linear correlation analyses suggested that this effect was a result of the reduction in renal function that occurs with increasing age.