We measured Asc.- (ascorbate radical) produced by the reaction of AscH- (ascorbic acid) with HO. or O2.- after irradiation in mouse. It is possible to measure Asc.- easily using ESR and a dialysis method in which Asc.- was collected at room temperature in the interstitial fluid through the dialysis membrane. After irradiation, Asc.- increased in both normal muscle and tumour tissues (SCC-VII) in proportion to the radiation dose. Asc.- increased after treatment with the agents H2O2 and FeCl2, while it decreased after treatment with SOD and catalase. These results suggest that the amount of HO. and O2.- produced is reflected in the Asc.- production. Also, the increase in Asc.- production when WR-2721 was administered prior to irradiation was less than for experiments in which irradiation only was performed. This method is useful for the following reasons. First, no special treatment, such as freezing of the sample, and no administration of noxious agents are necessary to measure Asc.-. Second, irradiation using a dose of only a few Gy shows an increase in production of Asc.-. Third, this method does not require removal of organs. Using this method, Asc.- can serve as an indicator of the amount of HO. and O2.- produced by irradiation in vivo.