This study was designed to investigate the histologic changes in goitrogen-induced thyroid growth of the rat. The animals were orally treated by 1% potassium perchlorate except the controls and were sacrificed in intervals ranging from 1 to 12 months. The thyroid weight increased progressively along the treatment and after 2 months showed a diffuse homogenous hypertrophy and hyperplasia of follicular cells, decreased amount of colloid and increased vascularity. After sixth month of treatment true nodules appeared with complex morphology.