Perchlorate ions ClO4(-), known to inhibit competitively the uptake of iodine by the thyroid, have been detected in drinking water in France as well as in infant formulae. A tolerable daily intake (TDI) has been established at 0.7 µg kg(-1) bw day(-1) based on the inhibition of iodine uptake. Due to this mechanism of action, the iodine status could strongly influence the biological effect of perchlorate. Perchlorate concentrations in water and infant formulae were measured and the exposure of children under 6 months of age calculated. It appeared that the TDI could be exceeded in some children. As the iodine status is not optimal within the entire French population, there appears to be a need to clarify the sources of perchlorate ultimately to decrease exposure.