A comprehensive review of the mammalian testis reactions under experiment, particularly with reference to the development of the conception that the scrotum of mammals is a thermoregulator for the testes, a temperature below that normal for the body being es sential for complete spermatogenesis in mammals that have a definite scrotum. Secondly, a review of the broader aspects of the internal secretion of the testis is attempted. The literature citations number 212. The paper is divided into topical sections (1) A general consideration of the testis in its relation to the scrotum, with a few extracts from older writers whose conception of the scrotum was more or less restricted to its possible evolutionary development. (2) Cryptorchidism, normal and experimental, is reviewed from the time of John Hunter to the present. The explanation of the structural conditions of un-descended testes is made clear by experimental work, and is found to be an effect of higher temperatures of the abdomen, rather than of faulty embryology. (3) The problem of vasectomy is reviewed, and an attempt is made to restore order amid the heterogeneity of results reported since 1823. Its place among modern surgical practices as a remedial means for old age, real or imaginary, is criticized. (4) Testis transplantation in mammals is reviewed and the newer developments in the field brought into line with the problem as a whole. The author reports his success in transplanting testes so that spermatozoa have developed in the graft. (5) In the light of the foregoing problems the functional significance of the scrotum is discussed. It is emphasized that the function of this newer mammalian structure is indispensable for the production of germ cells. (6) The literature on X-ray treatment and its sterilizing effects is treated. (7) New work on the effects of hypersensitive reactions and the deleterious effects on the testicle are discussed. (8) Many different results on testis studies are discussed[long dash]the effects of sickness and subsequent sterility; the effects of alcoholism and dietary deficiencies. (9) A brief review of the literal ture on the internal secretions of the testis and the genera-trends, ancient and modern, in this broad and complicated field. This field has dealt with many different phases, observational and experimental. It has been studied by one or more of the methods discussed in the former headings as well as in seasonal activity of animals, and by cyto-logical and embryological observations.