Abstract The recent advances in our knowledge of the function of the anterior pituitary lobe in relation to gonadic development and activity, have led to an investigation of the part played by the pituitary in controlling testicular growth and function. The understanding of the relation between the hypophysis and the gonads in the male is, in general, less advanced than for the female, in which this relation was first established. It has been recognized that any action on the testis may be exerted on two morphologically and physiologically different parts: (1) The tissue concerned with the elaboration of ripe spermatozoa, and (2) the interstitial tissue producing the internal secretion responsible for the development of some of the secondary sex organs and characters. The development in these two tissues does not always follow a parallel course, and it appeared possible that different factors were responsible for controlling their growth and functional activity. Consequently a large number of investigations have been made on the action of various extracts and substances upon the growth and secretory activity of the testis. It is not here intended to review the whole literature on this subject, but a number of the effects obtained may be briefly recapitulated:— (1) Numerous experiments have been recorded on the effect of specific hormonic factors. Following on the findings of Smith (1930), who showed that hypophysectomy resulted in atrophy of the testes and of the secondary sex organs, the action of various pituitary extracts was determined.