The present work studied the TSH response to TRH in T4- and T3-treated normal rats and in rats with depressed intrapituitary conversion of T4 to T3. Euthyroid normal rats were injected iv with a single dose of 6 μG T4/100 g body weight, 1 μg T3/100 g or normal saline. Another group treated with iopanoic acid (IOP) received 1 or 10 μg T4/100 g, 0.21 or 2.5 μg T3/100 g, or saline as control. Twenty min later 1 μg TRH/100 g was injected iv. Blood samples were drawn at times 0, 20 min (immediately preceding the injection of TRH) and 30 min (10 min post-TRH) for measurement of plasma T4, T3 and TSH. Other groups of IOP-treated rats were injected iv with 150 μCi of [3',5'-125I]T4; the hypophyses were removed at 30 min and homogenized in PBS buffer, extracted and chromatographed in tertiary amyl alcohol:hexane:ammonia. The control group of rats had a 13-fold increase in plasma TSH 10 min after the TRH injection, while rats treated with 6 μg T4/100 g had a 7.6-fold increase (P < 0.01 vs control increment) and rats treated with 1 μg T3/100 g had no change in TSH response as compared to controls. In IOP-treated rats injected with 10 μg T4/100 g there was a smaller increase in the TSH response to TRH as compared to control (P < 0.01), and a similar smaller response was seen in rats treated with 2.5 μg T3/100 g (P < 0.01 vs control). IOP-treated animals given 1 μg T4 or 0.21 μg T3/100 g had no significant changes in TSH response to TRH. It seems probable that T4 possesses an intrinsic hormonal capacity to regulate the secretion of TSH before it monodeiodinates to T3.