Pretreatment of rats with either dexamethasone or chlorpromazine, morphine, and Nembutal, though completely preventing the rise in plasma corticosterone and hence the release of ACTH otherwise associated with the intravenous injection of either saline or synthetic TSH-releasing factor (TRF) (200 ng), under ether or Nembutal anesthesia, failed to alter the plasma TSH response to TRF recorded 10 min after the injection. The plasma corticosterone response to the stress of the intravenous injection was of the same order in the animals injected with TRF as in the saline-injected controls. These findings fail to support the hypothesis of a competition between ACTH and TSH secretion at the pituitary level.