Besides abnormalities of the male genital tract, we must always consider the numerous factors in the female that may kill or weaken the sperms. Impediments to successful coitus may be present in the form of vaginal constrictions, abnormally short or small vagina, or other deformities. Hostile exudates, mechanical obstructions, and other factors may interrupt the progress of the sperms at any point in the tract. Although little is known definitely regarding disorders of the endocrines in the bull and their relation to reproduction, the work in human medicine and experimental researches upon laboratory animals warrant thoughtful consideration of these factors which are by no means insignificant. In the future, these glands will no doubt receive more and more attention in their relation to the genital organs and reproduction. CONCLUSIONS 1. The genital organs of the bull quite frequently undergo pathological changes, due to infection with the same varieties of microorganisms associated with genital infections in the female. 2. In all probability, these microorganisms are frequently eliminated with the semen and infect the female during copulation. 3. Past or present infection in the genital organs of all the bulls so far examined was evidenced by the presence of the fine connective tissue tufts and strands upon the tunica vaginalis, particularly that part covering the tail of the epididymis. 4. Lowered sexual capacity is, as a rule, accompanied by demonstrable changes in the semen. 5. A study of impotency and sterility includes not only a thorough study of the genital organs, but also those extrinsic factors which govern reproduction either directly or indirectly. 6. A thorough knowledge of the anatomy and physiology of the male genital organs is fundamental to a clear understanding of the problem. I am much indebted to Drs. W. L. and W. W. Williams for some of the material, and for helpful cooperation in the early part of the work; to Drs. C. M. Carpenter and R. R. Birch for many helpful suggestions; and to Dr. J. N. Frost and others who so kindly co-operated by placing at my disposal samples of semen and some of the genital tracts.