Puerarin and daidzin are the major isoflavone glucosides found in kudzu dietary supplements. In this study, we demonstrated that puerarin significantly improves glucose tolerance in C57BL/6J-ob/ob mice, an animal model of type 2 diabetes mellitus, blunting the rise in blood glucose levels after i.p. administration of glucose. In contrast, daidzin, the O-glucoside, had a significant but opposite effect, impairing glucose tolerance as compared to saline-treated controls. When they were administered i.p. with (14)C-glucose to C57BL/6J lean mice, puerarin inhibited glucose uptake into tissues and incorporation into glycogen, while daidzin stimulated glucose uptake, showing an opposite effect to puerarin. Puerarin also antagonized the stimulatory effect of decyl-beta-D-thiomaltoside, an artificial primer of glycogen synthesis, which increases (14)C-glucose uptake and incorporation into glycogen in mouse liver and heart. A liquid chromatography-tandem mass spectrometry procedure was used to investigate the metabolism and bioavailability of puerarin and daidzin. The blood puerarin concentration-time curve by i.p. and oral administration indicated that puerarin was four times more bioavailable via i.p. injection than via the oral route of administration. This may account for the increased hypoglycemic effect seen in the i.p. glucose tolerance test vs that seen orally. Our results suggest that puerarin is rapidly absorbed from the intestine without metabolism, while daidzin is hydrolyzed to the aglycone daidzein. The opposing effects of puerarin and daidzin on glucose homeostasis may have implications for the activity of dietary supplements that contain both of these isoflavonoids.