Compensatory thyroid growth in rats subjected to unilateral thyroidectomy (Tx) and superior cervical ganglionectomy (SCGx) performed ipsilaterally to the remaining lobe was about 2-fold that of rats subjected hemiTx alone, when assessed 7–15 days after surgery. HemiTx depressed serum T4 levels by 33% and increased serum TSH by 80% in control rats. Unilateral SCGx, which did not affect circulating T4 or TSH by itself, prevented hemiTx-induced changes of hormone levels. After hypophysectomy (Hpx) thyroid involution ensued. The subsequent hemiTx of Hpx rats did not affect Hpx-induced thyroid regression 7 days later, but abolished it 15 days later. At this time a factorial analysis of variance revealed lack of significant interaction between Hpx and hemiTx thus suggesting that a similar thyroid growth ensues after hemiTx regardless of pituitary intactness. However, both serum T4 and TSH levels were very low in Hpx rats regardless of whether one or two thyroid lobes were present indicating that thyroid growth in the absence of pituitary was not accompanied by increased secretory activity. SCGx performed ipsilaterally to the remaining lobe blunted the growth of the lobe in Hpx hemiTx rats. Moreover, the denervated thyroid lobe regressed in Hpx rats to values significantly lower than those of Hpx rats having intact sympathetic nerves both in the presence or in the absence of a contralateral thyroid lobe. SCGx did not modify the very low serum TSH and T4 levels of Hpx rats. These results indicate that (1) in rats with intact pituitaries ipsilateral SCGx amplifies the compensatory growth and increases T4 secretion of the remaining lobe after hemiTx; (2) in Hpx rats ipsilateral SCGx blunted the growth that follows unilateral Tx and induces further regression of the denervated lobe regardless of whether a contralateral lobe is present or no