SUMMARY: The rate of thyroidal iodide clearance has been measured repeatedly during the periods 2–20 min., 20 min.-2 hr., 2–10 hr. and 10–24 hr after i.v. administration of 131I Early measurements (after 2–20 min.) gave higher values than the estimates obtained at later, more conventional, periods. Treatment with carbimazole produced a slight decrease in the clearance when measured between 2 and 20 min., a more pronounced decrease when measured between 20 min. and 2 hr., and a decrease to virtually zero after 2 hr. It is suggested that the 2–20 min. measurements may correspond to the theoretical 'unidirectional' clearance, whereas later measurements, and especially those obtained after 2 hr., may correspond to the theoretical 'net' clearance. The 'net' clearances, expressed as a percentage of the 'unidirectional' one, may probably be used as an index of the proportion of iodide which, after trapping, remains organically bound in the thyroid. In normal persons the mean 2–10 hr. clearance was 55·4 ± 3·09 (s.e.)% of the value obtained between 2 and 20 min., suggesting that normally only about 55 % of the iodide trapped remains in the thyroid. In thyrotoxicosis and simple goitre the clearance values were high in absolute terms, but when expressed as a percentage of the 2–20 min. values, results obtained during the periods 20 min.-2 hr. and 2–10 hr. did not differ significantly from normal. In a patient with Pendred's syndrome with defective organic binding of iodide, the 2–10 hr. clearance was 7·4% of the 2–20 min. value.