Small highly active aberrant thyroid tissues were detected at various sites in thyroidectomized but not in normal guinea pigs of the Hartley and Zeimet strains by their ability to very actively accumulate I131. Histological study showed follicles lined with flat cuboidal cells and the lumens filled with colloid. Aberrant thyroids have been frequently detected as pathological growths at many different sites in man (1, 2). Studies utilizing serial histological examination of the thyroid area have indicated the presence of aberrant thyroids also in the guinea pig (3, 4, 5, 6), rabbit (3), dog (7, 8) baboon (9) and rat (10, 11). It seemed that a more detailed study of the distribution of aberrant thyroids could be made by the use of the sensitive I131 uptake technic.