The central dopaminergic system seems to have an inhibitory influence on the secretion of thyrotropin (TSH) both in humans and rats. This is documented by observations that in humans, especially in subjects with primary hypothyroidism, the dopamine precursor l-Dopa, dopamine receptor agonist bromocryptine, and dopamine itself decrease plasma levels of TSH and in some instances inhibit TSH secretory response to thyrotropin-releasing hormone (TRH). Conversely blockade of dopamine receptors leads to an elevation of circulating TSH. It is probable the dopaminergic drugs act on the pituitary thyrotrops rather than on the release of hypothalamic TRH. Analogous results were obtained in rats. In contrast, the noradrenergic system, studied mainly in rats, has a stimulatory influence on the secretion of TSH. This view is mainly supported by findings that acute interruption of noradrenergic neurotransmission causes a decrease of serum TSH levels and blocks the cold-induced stimulation of TSH secretion. The role of the central serotonergic system remains undecided because some experimental findings indicate that it has an inhibitory influence but others indicate an opposite function. The remaining transmitter systems have not been studied extensively enough to allow definite conclusions about their roles in the regulation of TSH secretion.