SummaryThe effects of thiouracil on turnover of I131 by the thyroid in immature and mature chickens were studied. Rate of uptake of I131 by the thyroid was significantly reduced and release rate of I131 from the thyroid was significantly augmented in thiouracil-fed birds. Rate of recycling of I131 from the metabolized thyroid hormone measured by accumulation of I131 by the thyroid from I131-labelled thyroxine injected was always a little less than rate of uptake of I131 by the thyroid. Both uptake and recycling of thyroidal I131 decreased with length of the period of thiouracil feeding. Rate of release of I131 from the thyroid increased with the lengthened time of thiouracil feeding. The thyroxine secretion rate expressed either per 100 g body weight or per bird measured by the thyroxine substitution significantly increased in thiouracil-fed birds compared with normal diet-fed birds. It also increased with length of thiouracil feeding. Use of thiouracil is not adequate for precise determination of thyroxi...