It is well established that there are two types of lymphocytes. One of these is rapidly dividing and short-lived. The thymic lymphocytes and germinal-center cells of the lymphatic tissues belong to this type. The other is produced at a slow rate and has a long life span. With respect to the incorporation of (3)H-thymidine into DNA, the former type is characterized by weak labeling autoradiographically, in contrast to intense labeling in the latter type. Experiments in mice have demonstrated that the lymphocytes of the thymus are capable of synthesizing DNA-thymine endogeneously from deoxycytidine which is circulating in a considerable amount and hence to be weakly labeled with injected (3)H-thymidine. In contrast, the majority of lymphocytes of the lymph nodes were found to be incapable of synthesizing DNA-thymine endogeneously from deoxycytidine and hence to be intensely labeled with injected (3)H-thymidine. When the pathway of synthesis of DNA-thymine endogeneously from deoxycytidine was blocked by the administration of 5-fluoro-2'-deoxycytidine, the labeling intensity of the lymphocytes of the thymus with (3)H-thymidine was greatly increased. Experiments in vitro with or without adding cold deoxycytidine to the medium provided further evidence for the utilization of deoxycytidine for the synthesis of DNA-thymine in the lymphocytes of the thymus.