Neurotensin, a basic peptide consisting of 13 amino acids, was originally isolated from bovine hypothalamus1 and subsequently from human and bovine intestine2,3. Radioimmunoassay and immunocytochemical studies have demonstrated that neurotensin-like immunoreactivity (NTLI) is distributed throughout the mammalian central nervous system (CNS)4,5 and the intestine, where it is localized to a distinct type of endocrine-like cell4,6. Neurotensin may serve as a neuro-transmitter in the CNS7 and as a hormone in the gastrointestinal tract6,8. In addition, NTLI has been detected in the rat pituitary gland4,5,9. Here we report that the NTLI present in the anterior pituitary gland of the rat is indistinguishable from synthetic neurotensin, using two chromatographic systems. It is localized in cells from which it can be released by depolarization in a calcium-dependent manner. The neurotensin content of the anterior pituitary in vivo was dramatically reduced after ‘chemical thyroidectomy’ induced by propylthiouracil treatment.