The effect of chronic administration of antithyroid drugs on the composition of thyroid proteins in dogs was investigated. After administration of thiourea for periods of 10 days to 4 months and also of ethylenethiourea for 6 months thyroid glands were examined morphologically by immunoperoxidase staining using anti-19S-thyroglobulin and anti-albumin antisera. After follicles were devoid of apparent colloid, the immunoreactivity of follicular cells to the 19S antiserum progressively increased with administration periods. A large number of follicular cells were also densely stained with the albumin antiserum. In contrast to the immunoreaction of 19S-thyroglobulin, which was detected in all follicular cells, the appearance of immunoreactive albumin was restricted to 60-80% of the follicular cell population even after several months' administration. Thyroid extracts after treatment with ethylenethiourea for 6 months were analyzed with gel chromatography, polyacrylamide gradient gel electrophoresis, and immunoblotting. Large-molecular-weight components, including 27S iodoprotein, were completely absent in the goitrous thyroids. On the contrary, small-molecular-weight components markedly increased in number and amount, although of these two or three bands alone were weakly immunoreactive to the 19S antiserum. Both 19S and 12S components were still significant products, showing an intense immunoreactivity for 19S-thyroglobulin after the chronic administration of goitrogen. Albumin markedly increased in amount and constituted 24.2% +/- 7.6% of total stained proteins, seven times as high as the value of normal controls (3.4% +/- 2.3%).