Resting plasma levels of growth hormone (GH) and corticosterone (B) were examined in the thyroid-deficient rat. Plasma GH was found to be lowered in thyroparathyroidectomized (TPTx) and 6-methylthiouracil (6MTU)-treated animals, but was normal in similarly treated animals given thyroxine (T4). Plasma B levels were not significantly different in sham-operated, TPTx, or 6MTU-treated animals. T4 elevated plasma B in sham-operated and TPTx animals, but had no effect in animals treated with 6MTU. These data suggest that thyroid hormone is necessary for the maintenance of the normal resting plasma GH level but does not affect the circulating B. In addition, the evidence suggests that 6MTU may have an effect on the adrenal cortex.