The increase of concentrations observed for many drugs when administered concomitantly with grapefruit juice was attributed to inhibition of cytochrome P450 enzymes by naringenin, the aglycone of the grapefruit flavonoid naringin. However, this explanation is equivocal, and formation of naringenin after ingestion of grapefruit juice has not been proved. We investigated renal excretion of naringin, naringenin, and its glucuronides after administration of 20 ml grapefruit juice (621 mumol/L naringin) per kilogram of body weight to six healthy adults. Urine was collected for 24 hours, and flavonoids were measured by HPLC in aliquots with and without glucuronidase pretreatment. Naringin or naringin glucuronides were not found. Naringenin and its glucuronides appeared in urine after a median lag-time of 2 hours and reached 0.012% to 0.37% and 5.0% to 57%, respectively, of the molar naringin dose. In additional investigations, low concentrations (< 4 mumol/L) of naringenin glucuronides, but neither naringin nor naringenin were found in plasma samples from previous grapefruit juice interaction studies, and metabolization of naringin to naringenin occurred during 24 hours of incubation (37 degrees C) in three of five feces samples tested. The data suggest that cleavage of the sugar moiety, presumably by intestinal bacteria, is the first step of naringin metabolism. Naringenin formation is thought to be the crucial step in determination of bioavailability of the compound, which undergoes rapid glucuronidation. The pronounced interindividual variability of naringin kinetics provides a possible explanation for some of the apparently contradictory results of drug interaction studies with grapefruit and naringin.