Glibenclamide, a hypoglycemic sulfonylurea, is extensively metabolized by the body and eliminated primarily in the form of its hydroxylated derivatives. The major metabolite, 4-trans-hydroxy-glibenclamide, is usually cleared rapidly from the bloodstream, but in certain pathological states (e.g. renal failure) blood levels of this product may increase. A protocol therefore was designed to test the hypoglycemic potency of this important metabolite in rats. Various quantities were injected intraperitoneally, and alterations in blood glucose concentrations were measured during a 5-hour period and compared to those in animals similarly treated with glibenclamide and in saline-injected controls. Using the dose capable of decreasing blood glucose levels by 30% (ED30) as a comparative index, it was observed that the metabolic has a marked hypoglycemic activity; though 6--7 times less potent than the parent drug, 4-trans-hydroxy-glibenclamide is nevertheless more potent than tolbutamide. Thus, while the glibenclamide metabolite probably has little influence on blood glucose when its clearance is normal, this product may exert marked effects if allowed to accumulate in the blood, as for example in renal failure. Finally, the role of such sulfonylurea metabolites should be taken into account when attempting to explain the occasional excessive and sustained hypoglycemia which occurs in some diabetic patients treated with these drugs.