The effects of thyroid hormone deprivation and restitution on growth hormone (GH) economy have been studied in the rat by means of a specific radioimmunoassay. The pituitary GH content and the plasma GH levels before and during stimulation with pentobarbital ("PB-test") were studied in male rats at different intervals after surgical thyroidectomy (T), and in T rats at different time intervals after the ip injection of 0.20, 1.75, and 5.0 mug thyroxine (T4) or 0.05, 0.10, 0.20 and 1.0 mug triiodothyronine (T3), all doses being referred to 100 g body wt. Pituitary GH content decreased very rapidly after T, a difference being shown at the end of the shortest time interval studied (24 h); 24 days after T, pituitary GH content was 0.3 percent or less of the pre-T level, the basal plasma GH was lower than in intact controls and an increase in plasma GH during PB-stimulation was no longer observed. When rats T for 30 days or longer were injected once with T4 or T3, pituitary GH content increased; basal plasma GH levels increased also and a positive response to PB was observed. An effect on pituitary GH content could be observed as soon as 2 h after the ip injection of 1.0 mug T3, or 6 h after 5.0 mug T4. The "latent period" was somewhat longer when lower doses of the hormones were used. Effects of a single 0.10 mug T3 dose could be detected within 12 h L-T3 appeared to be at least 9 times more potent in vivo tha T4, as assessed from the effect on pituitary GH. The mea-urement by RIA of changes in GH content of the rat pituitary may thus provide the most adequate parameter available at present (other than suppression of TRH-induced TSH release) for a biological effect in vivo of single doses of the thyroid hormones, a measurement clearly related to an important physiological role.