The present study was undertaken to examine whether long acting thyroid stimulator (LATS) induces refractoriness of thyroid hormone synthesis and thyroid hormone secretion to the stimulator in vivo. Male DDY mice fed with a low iodine diet and given 5 micrograms/ml of triiodothyronine (T3) in drinking water and libitum for 4 days were injected with 0.025 ml of LATS positive serum (1000%/0.25 ml in rhe McKenzie bioassay) ip every 24 h for 9 days. Groups of 5 mice were sacrificed before and 1, 3, 5, 7 and 9 days after the first injection of LATS for the determinations of serum thyroxine (T4) concentrations, the 1 h thyroid 131I uptake and thyroid weight. Control mice were injected with LATS negative pooled normal sera. Serum T4 concentrations elevated significantly 24 h after the 3rd injection of LATS and remained elevated until the end of the experiment. One hour thyroid 131I uptake elevated about 3-fold 24 h after the first injection of LATS. It further increased 24 h after the 3rd injection of LATS to 10-fold of the value for control animals and stayed elevated at this same level for the remainder of the study. These results indicate that stimulating effects of LATS on thyroid hormone synthesis assessed by thyroid 131I uptake and thyroid hormone secretion assessed by serum T4 concentrations were not diminished by the prior administration of the stimulator. These findings suggest that LATS does not induce refractoriness of either thyroid hormone synthesis or thy roid hormone secretion to the stimulator in mide in vivo.