SummaryThe purpose of the present investigation was to determine whether rat thyroid fucokinase is responsive to hypophysectomy and to administration of thyroid stimulating hormone (TSH). Prior to measurements of fucokinase activity in thyroid extracts, any potential low-molecular-weight effectors were removed from the extracts by a molecular sieving procedure. With this procedure, thyroid fucokinase activity in hypo-physectomized rats was found to be 55% lower than in intact rats. Injections (i.p.) of TSH into the hypophysectomized rats, however, restored the activity lost upon hypophysectomy to levels that were not significantly different from those of intact animals. Also, when intact rats were given injections (i.p.) of TSH, thyroid fucokinase activity was elevated significantly above that found in intact animals given injections of carrier only.