A rate of incorporation of 3H-thymidine, 3H-uridine and 14C-glycine into the nucleoproteins of bone marrow, spleen, thymus and ileocecal lymph nodes was studied in BALB/c mice with hypothyrosis developed after administration of 1% KClO4 within a month. Distinct hypoplasia of bone marrow and spleen was found in the experimental animals; the synthesis of nucleoproteins was most impaired in bone marrow. The dependence of hemopoietic tissues on thyroid hormones was decreased as follows: bone marrow > spleen > thymus > lymph node. After subcutaneous administration of triiodothyronine (T3) in a dose of 50 mg/kg daily within 3 days of synthesis of nucleoproteins was considerably stimulated and the amount of cells was increased in bone marrow and spleen. Less distinct stimulating effect of T3 was observed in the thymus and lymph nodes of these animals.