Abstract
In order to examine the hypothesis, so ably enunciated by the Steinach school, that senility is a phase of life that is attended by, perhaps even caused by, the inefficient functioning of one or more of the components of the endocrine system, the writer set out upon a series of attempts to evoke rejuvenation in the aged fowl. In a previous communication (Crew, 1925) an account was given of an unsuccessful attempt to produce this result in the aged cock by means of unilateral vasoligation. In this paper the results of thyroid administration to aged birds of both sexes are reviewed.