Abstract
The adm. of thyroxin to [male] rats for 10 days caused marked thyroid atrophy whether the pituitary stalk was severed or intact. Other physiological effects of thyroxin were also unchanged. The thyrotropic function of the anterior hypophysis is evidently for the most part controlled humorally by variation in the organism''s thyroxin level without mediation of the hypo-thalamo-hypophyseal pathways.