Male rabbit germ cells were labelled by intratesticular injection of [3H]-thymidine (3H-T). In sperms of control animals, radioactivity was first demonstrated between the 40th and 43rd day after labelling, corresponding to preleptotene spermatocytes. In rabbits treated with 22.5 mg/kg methylmethane sulfonate (MMS), significant radioactivity was shown in sperms collected from day 19 ownwards. These cells derived from spermatocyes and early spermatids at the time of labelling. 3H-T incorporation into these cell populations represents unscheduled DNA synthesis (UDS), a repair process initiated after chemical damage of germ cell DNA. After i.v. injection of 20 mg/kg cyclophosphamide, an alkylating agent that must be activated, labelled sperms were found 28--37 days after treatment. This shows that UDS took place in spermatocytes during the pachytene and zygotene stages. Adriamycin (1.0 and 3.0 mg/kg) induced UDS during pachytene and zygotene stages of spermatogenesis. Sperm counts decreased during spermatogonial stages by a factor of about ten in cyclophosphamide and adriamycin treated rabbits. It was not changed after MMS-treatment.