Forty-nine euthyroid subjects were studied on two successive days to evaluate the constancy of thyroidal I131 uptake. The thyroidal uptake of I131 and the urinary excretion of iodide (I127) were measured three and twenty-four hours after giving I131 tracers. Six patients had day-to-day changes in 24-hour I131 uptake of more than 10 percent of the administered tracer; 3 had variations of 3-hour I131 uptake of more than 6 percent. No correlation between the variability of the thyroidal uptake of I131 and the amount of iodide excreted was found when the daily excretion was less than 500 [mu]g; when more than this amount was excreted, variability in thyroidal I131 uptake from one day to the next was no longer encountered. Thus, when an apparent discrepancy between the clinical picture and the thyroidal uptake of I131 is observed, day-to-day variation of this index of thyroid function should be considered among the possible causes of such a finding.