DNA repair synthesis of tobacco seeds induced by strong pulsed electron and Ar,+, ion beams was studied with ,3,H-TdR incorporation method and DNA inhibitor treatment. It was found that there were much more incorporated ,3,H-TdR during 28~44 hours of germination in implanted seeds than that in normal seeds, and there were two DNA synthesis peaks during early stage of germination(within 60 hours) in the implanted seeds, but only one peak in normal seeds. The first DNA synthesis peak of the irradiated seeds was strongly suppressed by caffeine which is an inhibitor of DNA repair synthesis, the second one was strongly suppressed by hydroxyurea which is an inhibitor of DNA replication. These results indicated that both of strong pulsed electron and Ar,+, ion beams could induce the damadge and repair of DNA in tobacco seeds. DNA repair synthesis started after 28 hours of germination and reached peak at the 36 th or 40 th hour in irradiated seeds. The peak of DNA repair synthesis was 12 or 16 hours earlier than that of DNA replication.