We propose that when skeletal muscle regenerates after injury, myogenic precursor cell replication commences earlier if the muscle has undergone a recent previous cycle of regeneration. To test this a series of muscles in mice were crush-injured and allowed to regenerate. Then either 7 or 28 days after the initial injury, the muscles were reinjured. The onset of DNA synthesis in myogenic precursor cells was determined by injecting each mouse once with tritiated thymidine at a specific time after the second injury. All reinjured muscles were left for 10 days to regenerate to the myotube stage. The presence of autoradiographically labelled myotube nuclei indicated that these nuclei were the progeny of myogenic precursors labelled at the time of injection of tritiated thymidine. Thus the onset of muscle precursor cell replication was determined. A similar series of experiments was conducted on muscle autotransplants, injured at 28 days after transplantation and sampled 10 days later. In none of these situations did muscle precursor cell replication occur earlier than in control muscles, which were injured only once, where myogenesis commenced between 24 and 30 h after injury. The autoradiographic data do not support our hypothesis. We conclude that the increased numbers of muscle precursors reported (by various authors) after repeated cycles of regeneration, are not due to the earlier initiation of DNA synthesis in myogenic preoursor cells.