To examine whether the injection of bovine TSH (bTSH) produces maximal radioactive iodine uptake (RAIU) in lung metastases in patients with differentiated thyroid cancer, 10 patients were studied 12 times. In 10 of these studies, an initial RAIU measurement was performed immediately after 3 injections of 10 IU bTSH given immediately after T3 withdrawal. Another RAIU measurement was performed 7-19 days after T3 withdrawal. Uptake increased in all patients even when it was clearly detectable immediately after bTSH stimulation. Thus, 3 days of bTSH stimulation in these patients did not lead to maximal 131I uptake, and it could only be reached after prolonged endogenous TSH stimulation. bTSH was not injected in the 2 other patients, in whom 6 RAIU measurements were carried out. Radioiodine uptake increased with time in both patients. It appears that both the level of endogenous TSH and the length of stimulation play a determining role in RAIU. This might explain why 3 days of bTSH stimulation are insufficient to elicit maximal 131I uptake.