A reciprocal relationship occurs between the level ofthyroxine in blood and the intensity of TSH secretion from the anterior pituitary. If the synthesis and release of thyroxine within the thyroid gland is in some manner (as a result of iodine deficiency, goitrogens, enzyme defects) inhibited or retarded, the blood thyroxine level will eventually decrease and, as a consequence, the TSH release rate from the pituitary will increase above normal. This, in turn, results in hypertrophy and hyperplasia of the thyroid. Under these conditions a hyperplastic thyroid may prove to be an indicator of an inadequate rather than an increased thyroxine secretion rate. It is known that thyroxine is essential in maintaining efficient milk secretion (Turner 1968). Furthermore, it is quite possible that a greatly increased TSH secretion may interfere with the synthesis and release of other anterior pituitary hormones needed for maintaining an optimal secretion of milk.Swett et ai. (1955, p. 31) observed significant positive correlation between the relative thyroid weights and the milk and butterfat yields in Jerseys but not in Holsteins and Guernseys. In the study of Kossila (1967, p. 73), the thyroid weight of the cows was positively and significantly correlated with the body weight and age, while it was negatively and nonsignificantly correlated with the milk yield, when the body weight and age were held constant. However, compared to the weight of the thyroid, the epithelial tissue content of the gland is probably a more reliable indicator of the thyroxine secretion rate or of the intensity of the TSH stimulation on the thyroid gland (Tala 1952, Uotila & Kannas 1952, Lamberg 1953. The purpose of this study was to find out whether there were any significant correlations between the absolute amount of epithelial tissue contained in the thyroid gland of the cows and their body weight, age, and capacity of milk production.