A simple and rapid way to measure the concentration of iodide in urine with an iodide-selective ion electrode was described. Potentiometric equilibrium was attained in less than 5 min, and a linear calibration curve was obtained over the potassium iodide (KI) concentration range of 10(-2) to 10(-6) M. The coefficients of variation ranged from 6.2 to 10.0% within assay, and 5.4 to 14.4% between assays. The serial dilution of 3 urine samples with different concentration of iodide showed good linear correlations passing through zero. In practice, the chloride ions in urine did not cause serious errors in the measurement of iodide at molar ratios of chloride ion to iodide up to 2 X 10(4). A good linear correlation was obtained between iodide concentrations in urine determined by the electrode method and by the conventional chemical method (r = 0.92). A linear correlation was also observed between the iodide concentrations of 24 h collected urine and those of single morning urine (r = 0.91). The normal iodide content in single morning urine specimens from 127 Japanese people was 5.3 to 62.0 X 10(-6) moles/g creatinine.