1. After one oral dose of 100 mg triamterene plasma levels of triamterene (TA) and its metabolites hydroxytriamterene (OH-TA) and hydroxytriamterene sulfuric acid ester (OH-TA-ester) were studied in patients with normal and impaired renal function and in patients submitted to hemodialysis. Triamterene and its metabolites were also determined in the urine and the dialysate, respectively. 2. Hydroxytriamterene sulfuric acid ester (OH-TA-ester) representing a phase-II-metabolite of triamterene was found to yield plasma concentrations which were always higher than those of native TA. 3. In decreased renal function the elimination half-lives (t 1/2) of TA and OH-TA-ester were increased yielding elevated plasma levels of TA and its metabolites. The data of t 1/2 for TA and OH-TA-ester correlated with the creatinine clearance in an almost hyperbolic fashion. 4. Dialysance values of about 2 to 4 ml/min were determined for TA and OH-TA-ester.