The effects of thyroid hormone deprivation and of subsequent replacement therapy on growth hormone (GH) secretion were investigated in unrestrained unanesthetized rats. Male rats were thyroparathyroidectomized (TPTX) 5 weeks prior to plasma sampling for GH assay, or to decapitation for evaluation of hypothalamic somatostatin (SRIF) content and in vitro SRIF and GH release. Thyroid hormone deprivation suppressed pulsatile GH secretion as well as GH release induced by clonidine (150 µg/kg). Treatment of TPTX rats with small doses of triiodothyronine (T3) restored an episodic pattern of GH secretion, but with lower peak values than controls, as well as the GH response to clonidine. Thyroid deprivation induced a 92-fold decrease in GH release from the pituitary; however, the ratio between GH release and GH content was similar in TPTX and normal rats, and human pancreatic growth hormone-releasing factor (GRF) (3 · 10–8M) was still able to stimulate residual GH release by hemipituitaries from TPTX rats in a manner similar to that in euthyroid controls (295 and 254% stimulation, respectively). Thyroid deprivation or T3 replacement did not modify SRIF content in the hypothalamus or other brain structures tested. The capacity of K+ depolarization to release SRIF in vitro from the hypothalamus was not modified by TPTX. These findings indicate that thyroid hormones are necessary to maintain both pulsatile and induced GH secretion in unanesthetized rats. In addition they suggest that impairment of GH secretion in thyroidectomized rats does not depend upon changes in the hypothalamic SRIF regulation of the hormone but could be dependent on a defect in GRF release and/or, most probably, GH synthesis directly at the pituitary le