The importance of several amino acids (glycine, L-glutamic acid, L-serine, taurine and β-alanine) in the regulation of the stimulated secretion of TSH was studied in male rats using both peripheral and central administration of the amino acids. Glycine (10–200 mg/kg i.p.), L-glutamic acid (10–500 mg/kg i.p.) and L-serine (500 mg/kg i.p.) decreased significantly the cold-induced TSH secretion whereas β-alanine (1–500 mg/kg i.p.) and taurine (10–100 mg/kg i.p.) were not effective. The effect of L-glutamic acid (100 mg i.p.) was partially antagonized by bicuculline (1 mg/kg i.p.) but not by picrotoxin (1 or 2 mg/kg i.p.). Only glycine (50 and 100 mg/kg i.p.) inhibited the TRH-stimulated TSH secretion. When the intracerebroventricular route was used, L-serine (50 µg/rat) decreased the TSH could response whereas glycine and L-glutamic acid (1–50 µg/rat) had no clear effect. We conclude that glycine, glutamate and serine inhibit the cold-induced TSH secretion in the male rat. The action of serine and glycine is possibly mediated through the periventricular hypothalamus and the anterior pituitary, respectively. The inhibition caused by glutamate seems to be partially mediated through the bicuculline-sensitive GABA receptors in the hypothalamus. Taurine and β-alanine play no role in the control of rat TSH secretion.