As a model for chemical DNA repair, reduction of guanosyl radicals in the reaction with cysteine or the dipeptide cysteine-glycine has been studied by time-resolved chemically induced dynamic nuclear polarization (CIDNP). Radicals were generated photochemically by pulsed laser irradiation of a solution containing the photosensitizer 2,2'-dipyridyl, guanosine-5'-monophosphate, and the amino acid or peptide. In neutral and basic aqueous solution, the neutral guanosyl radical is formed via electron or hydrogen atom transfer to the triplet excited dye. The rate constants for reduction of guanosyl radical were determined by quantitative analysis of the CIDNP kinetics, which are sensitive to the rates of fast radical reactions. The rate constants vary from (1.0 ± 0.3) × 10(7) M(-1) s(-1) for the thiol form of cysteine to (1.6 ± 0.2) × 10(8) M(-1) s(-1) for the thiolate anion. These values are comparable with corresponding rate constants for reduction of neutral guanosyl radical by tyrosine.