THE presence of proteolytic substances in the thyroid gland was first demonstrated by the digestion of a gelatin film with follicular colloid, obtained by microdissection from rat thyroids (De Robertis, 1941). Later, whole thyroid extracts were shown to exhibit hydrolytic activity at acid pH against edestin (Dziemian, 1943), and hemoglobin (Kamner et al., 1950; Weiss, 1953). The proteolysis of thyroglobulin has been accomplished with thyroid extracts from dogs and swine (Roche et at., 1950; McQuillan and Trikojus, 1953). Recently a partially purified fraction of hog thyroids has been prepared, which is rich in proteolytic activity against hemoglobin and thyroglobulin (McQuillan and Trikojus, 1953). In the present study, the hydrolysis of rat thyroglobulin by thyroidal enzyme (s) has been investigated. Some of the conditions under which proteolytic activity occurs have been examined and the products of proteolysis have been evaluated. The method has differed from that of previous workers primarily in that ...