Nineteen infant and thirteen pregnant chimpanzees have been tested. The first experiments revealed that the uptake of radioiodine by the thyroid in two one year old chimpanzees was already very low without the administration of stable iodide. Subsequent determination of the urinary iodide levels showed that the daily output of the chimpanzees was 40 times normal, indicating the presence of an unknown source of iodide exposure. This source proved to be an iodide containing detergent used for disinfecting the housing of the chimpanzees. The thyroidal uptake of radioiodine in the two infant chimpanzees was 2.3% and 1.3% in this situation (October 1988). As expected, these values could hardly be reduced by adding extra iodide. After these findings the iodide containing agent was replaced and the excretion of iodide reached normal values. Four months later the thyroidal uptake of iodine 123 had increased in both animals, to 11.7% and 10%. Upon the administration of a dose of 0.5 mg potassium iodide /kg/day thyroidal uptake was effectively blocked. There was no identification of severe thyroid disorders, however, in either of the 2 animals.