Female rats were killed 15 days, 2 months, and 4 months after surgical thyroidectomy that was followed by injection of 100 microCi 131I. The concentrations of T3 and T4 were measured in tissues (liver, kidney, brain, heart, and hindleg muscle) specific RIAs. Results were compared to those found in intact rats. Thyroidectomy resulted in severe hypothyroidism by 2 and 4 months after the operation, as assessed by undetectable levels of T4 and T3 in unextracted plasma, high circulating TSH, hypothermia, stasis of body weight increase, and depletion of pituitary GH content. Concentrations of T4 and T3 in plasma, as determined after extraction and concentration, were very low, being less than 5% of the normal value by the earliest observation period (15 days). In contrast, although tissue concentrations and total organ contents also decreased after thyroidectomy, they were still clearly detectable 4 months after thyroidectomy. The rates of decrease of T4 and T3 concentrations in most tissues were markedly slower than expected from their rapid decrease in plasma. Some tissues still contained 20% of the normal level 2-4 months after ablation of the thyroid. Tissue levels of thyroid hormones were hardly detectable in rats thyroidectomized 6 months before, having decreased in most tissues to less than 5% of the normal value. Several animals from this group had died. It is concluded that tissues from severely hypothyroid thyroidectomized rats may contain higher concentrations of T4 and T3 than previously thought. The idea that thyroid hormone is not essential for life, based on the assumption that thyroidectomized animals survive without thyroid hormones, might have to be reevaluated.