The repair activities and the reaction mechanisms of two phenylpropanoid glycosides (PPGs), isolated from a herb used in Chinese folk medicine, towards the oxidizing hydroxyl radical adduct of poly G were studied with a pulse radiolytic technique. On pulse irradiation of a nitrous oxide saturated 4 mM aqueous solution of poly G containing one of the tested compounds, the transient absorption spectrum of the hydroxyl radical adduct of poly G decays with the formation of that of the phenoxyl radical of the tested compound within several tens microseconds after the electron pulse irradiation. The results indicated that there was a repair reaction between the hydroxyl radical adduct of poly G and the compounds tested. The rate constants for the repair reactions of poly G hydroxyl radical adducts by verbasicoside and cistanoside C were determined to be 1.4 and 1.0 x 10(9) M(-1) x s(-1), respectively.