Male chicks of an egg-laying strain were injected with 0, 1, 2 or 4 micrograms/100 g b. wt thyroxine s.c. daily from 7 to 12 weeks of age. Increasing dose suppressed testicular development and puberty was completely blocked by the highest level. Live weight gain was enhanced by thyroxine treatment. Similar treatment of females prevented normal development of the ovaries at puberty.