In order to study the ontogeny of TSH regulation in the rat, we have compared TSH secretion in neonatal and adult rats after treatment with morphine, TRH and apomorphine, clonidine, and exposure to cold. Apomorphine attenuated exogenous TRH-induced TSH release in both neonatal and adult rats. Morphine suppressed TSH levels at every age tested. Although clonidine and cold exposure elicited TSH secretion in adults, neonatal rats did not respond to either treatment. These findings suggest that opioid and dopaminergic controls of TSH release mature before central noradrenergic regulation in developing rats. This lack of noradrenergic control may account for the absence of the response to cold exposure in the neonate.