Neonatal administration of MSG leads to a syndrome of endocrine dysfunction characterised by reduced growth, obesity and hypogonadism. The aim of the present investigation was to gain information on the structure and function of the pituitary-thyroid axis in MSG-treated rats. Neonatal Wistar rats received an s.c. MSG (4 mg/g body weight) or hyperosmotic saline (controls) on days 2, 4, 6, 8 and 10 of life. Histological and morphometrical studies were carried out on the thyroids of rats during the 4th month of life. Plasma TSH, T3, and T4 were measured by RIA kits. MSG-treated rats showed stunted growth, obesity and decreased pituitary weight. MSG administration resulted in increases in thyroid weight, absolute volumes of epithelium, colloid and stroma, and blood T3 level while T4 level remained unchanged. In enlarged thyroid gland, percentage fractions occupied by epithelium, colloid and stroma were similar to those observed in control rats. The results obtained suggest that the rat hypothalamic centres involved in regulation of the pituitary-thyroid axis are slightly affected by neonatal MSG treatment.