Changes in distribution properties of T lymphocytes from lymphoid organs of normal rats 3H-uridine labeled in vitro and i. v. injected to tumor-bearing recipients were studied at different stages of tumor growth and rejection. Comparisons were made with T cell migration from normal donors to syngeneic recipients. A temporary decrease in the ability of normal spleen T lymphocytes to migrate into the spleen of tumor-bearing recipients at early stages of tumor growth was in correlation with their enhancement in trapping by the liver of this recipient. At this stage the migrations into tumor-draining lymph nodes and peripheral blood were rather pronounced. The growing tumor evoked an increased migration rate of donor spleen cells into each of the recipient organs studied except for liver. Tumor rejection potentiated the distribution mainly in the draining lymph node and liver. Normal lymph node T lymphocytes migrated at the beginning of tumor growth into spleen, node and liver of tumor-bearing recipients. A sharp decrease of distribution in spleen at the time of maximum tumor growth was compensated by an enhanced migration into draining nodes, peripheral blood and liver. This enhancement was seen also during tumor rejection. Distribution of normal blood T lymphocytes into spleen and peripheral blood of tumor-bearing recipients was equal to normal values. There was only a transitional reduction in their migration rates into draining nodes caused by tumor growth. Migration rates of thymocytes in tumor-bearing recipients remained unaltered. The results presented indicate that the distribution pattern of i. v. injected labeled T lymphocytes from normal donors into lymphoid organs of tumor-bearing recipients was determined by changes in the immune status of the recipients brought about by tumor development.