8-Hydroxydaidzein (8-OHD), which is produced during the processing of fer-mented soybean products, has a potent antioxidant activity in vitro. There is no information regarding the absorption and excretion of this isoflavone, including its antioxidant effect in vivo. In this study, rats were administered a single oral dose of 8-OHD (20mg/kg body weight), and the blood, liver, kidney and urine were collected at specific intervals up to 18h after dosing. Free 8-OHD in each tissue was directly determined by using HPLC with electro-chemical detection, while its conjugates were detected after the treatment with β-glucu-ronidase and sulfatase. The total 8-OHD in liver reached a high level (9.4 nmol/g) at 1h after dosing, and maintained the relatively high concentration up to 10h. Most of the 8-OHD was present in free form in liver, while the majority of 8-OHD in plasma was conju-gated. This suggests that free 8-OHD in liver is successively converted to glucuronide and/or sulfate and the conjugated 8-OHD is released into the blood. The maximum level of total 8-OHD in plasma or kidney was observed within the first 2h after the oral administration. The level of 8-OHD in these tissues gradually decreased within the further experiments. Excre-tion of the 8-OHD in urine began to rise at 1-2h interval. The mean urinary excretion rate of 8-OHD showed a higher level at 2-4h and 4-6h intervals, while the 8-OHD levels at these intervals in plasma or kidney more rapidly decreased. The cumulative recovery of 8-OHD in the urine over the 0-18h interval was about 36% of the dose. In addition, the liver homogenate from rats killed at 1h and 2h after dosing, which contained a higher level of free 8-OHD, showed a significantly lower susceptibility to lipid peroxidation induced by AAPH or Cu²⁺ than that at Oh (pre-administered rats). These results suggest that 8-OHD was relatively easily absorbed into rats and might exert its biological activities in vivo, including the antioxidant effect.