Thyroid function was investigated in adult male rats following the use of experimental procedures which inhibit the activity of serotoninergic neuron system. Pharmacological blockade of the biosynthesis of serotonin by repeated administration of p-chlorophenylalanine (pCPA), or interruption (by Halasz knife) of the serotoninergic pathways of the brain stem which terminate on hypothalamic nuclei equally resulted in an augmentation of the following parameters of hypothalamo-hypophysial-thyroid activity: T/S [thyroid/serum 131I] ratio, pituitary and blood TSH [thyrotropin] levels and blood thyroxine concentration as well as TRH [thyroliberin] content of the hypothalamus. The central nervous serotoninergic neuron system plays an inhibitory role in the regulation of TSH secretion, presumably acting upon the hypothalamus, thereby inhibiting hypothalamic TRH secretion.