Triamterene, a diuretic agent, has been reported to cause megaloblastic anemia in some patients. Because this drug is a pteridine derivative, we investigated its effect on folic acid absorption in the rat jejunum. In an in vivo intestinal loop method, triamterene inhibited the intestinal absorption of folic acid in a dose-dependent fashion, with 50% inhibition of systemic absorption occurring at a luminal concentration of 0.01 mmol/L of triamterene. Kinetic analysis using the influx chamber method demonstrated that triamterene is a competitive inhibitor of intestinal folate transport, with a Ki of 0.125 mmol/L. Because therapeutic doses can result in luminal concentration of the drug approximating or exceeding the Ki, the interaction between triamterene and folate absorption is potentially of clinical interest.