Radioactive wires and other linear sources are currently being used in clinical trials as endovascular brachytherapy to prevent restenosis after percutaneous transluminal coronary angioplasty A new concept is the use of a liquid-filled balloon containing a beta-emitting radioisotope. A major advantage is optimal delivery of the radioactivity to the vessel wall. Rhenium-188 (¹¹⁸Re) is a high-energy beta-emitter that is routinely available from a ¹⁸⁸W/¹⁸⁸Re generator in liquid form. Since ¹⁸⁸Re-perrhenate could be released in the unlikely event of balloon rupture, we investigated whether, in analogy to pertechnetate, subsequent use of perchlorate can reduce the uptake of perrhenate in the thyroid. We performed static (n = 9) and dynamic (n = 11) thyroid scintigraphy with ⁹⁹Tc^m-pertechnetate to estimate the overall reduction in activity within 30 min and the washout from the thyroid after oral administration of 600 mg perchlorate (T). In two patients, ¹⁸⁸Re was injected to estimate the whole-body distribution and the discharge of thyroid activity after perchlorate use. Based on MIRD Dose Estimate Report No. 8 (valid for ⁹⁹Tc^m-pertechnetate), the radiation burden was calculated for intravenous administration of ¹⁸⁸Re and competitive blocking with perchlorate. In 20 patients, ⁹⁹Tc^m uptake by the thyroid was reduced by 85% within 30 min by perchlorate. The mean (±S.D.) washout rate (T^1/2) was 8 ± 2 min in 11 patients. Perrhenate showed a whole-body distribution similar to that of pertechnetate and the thyroid activity could be displaced (T^1/2) = 6.3 and 9.3 min, respectively) by oral administration of perchlorate, with reductions in uptake of 83% and 75% within 30 min, respectively. Whole-body scanning demonstrated no regional accumulation of ¹⁸⁸Re-perrhenate with excretion by urine. Dose estimates gave an effective dose equivalent of 0.42 mSv MBq^-1which decreased to 0.16 mSv MBq^-1 after perchlorate blocking. ¹⁸⁸Re has favourable properties for endovascular brachytherapy via a balloon catheter and, in the unlikely event of balloon rupture, whole-body radiation can be reduced to 38% by subsequent oral administration of perchlorate. (