The incorporation of orotic acid-C/sup 14/ was investigated in C57BL mouse thymus DNA and RNA after local irradiation with 300 r, and in liver regenerating after partial hepatectomy. The effect of DNA synthesis in regenerating liver on DNA synthesis in regenerating thymus was further investigated by arranging the time sequence of the irradiation of the thymus and of the partial hepatectomy so as to make maximum DNA synthesis in liver coincide with maximum DNA synthesis in thymus. It was observed that, under these conditions, the incorporation of the precursor into the irradiated thymus was reduced 40%, suggesting a competition between liver and thymus for factors necessary for regeneration. (auth)