The fate of radioiodine injected into white-throated sparrows (Zonotrichia albicollis) collected in November, or weaver finches (Euplectes afer taha) collected in February or May, was followed. The thyroid of the white-throated sparrows accumulated an average maximum of about 60% of the injected dose within 6 hours. It was unusual in the high proportion of monoiodotyrosine formed. The February weaver finches concentrated 73% of the injected I131 in their thyroids, the May finches 57%. Additional dietary iodine did not significantly affect the thyroidal uptake. Thyroxine given to Euplectes did not appreciably affect the distribution of I131 in the different chemical fractions in the thyroid, but reduced uptake almost 100-fold from 57% to a maximum of 0.99%. Both species formed thyroxine as well as triiodothyronine. The proportion of administered radioiodine found as thyroxine in the thyroid of the weaver finch was unusually small, 10.7% in February and 2.4% in May, although in white-throated sparrows it was about 24.3%.