We employed single cell gel electrophoresis to analyze the kinetics of DNA repair in nuclei isolated from tobacco plants exposed to ethyl methanesulfonate (EMS), N-ethyl- N-nitrosourea (ENU) and γ-radiation. DNA repair was measured as the reduction of the tail moment values as a function of time after the mutagen treatment ended. DNA damage in leaf nuclei of EMS-or ENU-treated tobacco plants persisted over a 72 h recovery period. However, a reduction of the SCGE tail moment values in nuclei isolated from leaves was observed over a 4-week period of recovery. Newly emerged leaves expressed a lower level of DNA damage due to more efficient repair and/or dilution of initial DNA lesions during cell division. After 24 h recovery, leaf nuclei from cells exposed to 20 or 40 Gy of γ-radiation expressed complete DNA repair. These data indicate that DNA lesions induced by alkylating agents are not readily repaired and persist beyond 4 weeks. Enzymes necessary to repair γ-induced DNA lesions are fully functional in non-replicating leaf cells and single and double strand breaks are rapidly repaired.