SUMMARY: Rat chromosomes prepared from thyroid cells grown in culture did not differ from those described for other tissues of this species. The incidence of thyroid cells with abnormal chromosomes was not increased by giving propylthiouracil to rats for 15 weeks. The thyroid chromosomes were damaged by injection of 1–25 μc of 131I. The damage persisted in the unstimulated thyroid for 15 weeks. Administration of propylthiouracil to rats after 131I caused significant reduction in the frequency of thyroid cells with radiation-damaged chromosomes.