Administration of thyroid to caterpillars of Lymantria dispar did not cause distinct changes in duration of larval or pupal periods. There was distinct diminution in the chrysalid weight, but no constant negative correlation between pupal weight and duration of larval life. Lugol''s solution (I in KI) had no distinct influence on pupation rate of caterpillars of Pieris brassicae but diminished the pupal weight. Mortality was not increased by either I or thyroid feeding.